Look at thickness and split if necessary to reduce width of slices.

Cut slices so they stick to the back off the knife and when it starts to fill up you put the back of the knife toward the bunch and drag your blade downwards to make it release. This is because this is a time-consuming cutting task and that saves you some hassle at least.

Transfer to tray and put knife and cutting board into the dirty zone like your sink, because it has residue that does not easily rinse off by washing off and will pollute mushroom and onion.

Cut off sides of onion, split and cut thin slices, but not too thin either. After cutting separate them with your fingers to not make them fry together.

Put baguette in oven and turn on timer. If the baguette like this is taking 10 minutes in the oven, you should time it so that when there is about 5-6 minutes left you start frying the meat, to get fresh bread which might even still be a bit warm by the time you eat it.

Cut mushroom thin slices, let humidity evaporate low heat and let them get dry - important no oil or no other pan liquid added.

Fry onion a bit, no caramelization. Add when finished to pan with mushroom so you have roughly 50/50-ish-looking mix. Save rest as leftovers.

Divide meat 50/50-ish and add to pan which is hot with oil - you want to stand and wait until the pan is so hot you can hear and see it sizzles immediately after putting in. But even that might not be hot enough for Maillard effect which OFC is optimal, but not highly necessary as it is extremely difficult to achieve with so thin slices before they dry out with standard home cooking setup, but if it doesn't sizzle at least the meat will become white and dull. If done correctly you might get sum.

Cook and stand and wait until you should flip.

Water and bubbles like this are what you at least will get unless it is hot enough, but there is also the reverse need of the cheese coming that makes too high heat not good too, so it's not an amateur move. Just drain it by gathering the meat on one side and holding the spatula in front to let the water and most of the oil run out (and yes, one not so beautiful day your dinner will go straight in the sink using this trick).

The meat is starting to get fully fried on both sides - that's when you gather it on one side. Thing is that the meat will still cook a tad while the cheese is melting, so optimally it is still a tad juicy inside the bits. Then add that much cheese - yes, almost def you will need more than you think.

Start folding the cheese into the meat with the spatula to make sure the cheese sticks as little as possible, also it is extremely important to keep the cheese moldable so it does not start stiffening and getting crust as the crust detracts extremely much from the overall taste as crusty cheese has such an incredibly strong taste profile and it also makes it taste lowbrow.

Add salt and pepper and the onion and mushroom mix a tad before it is done. Then gather on the side for transferring into the baguette with a big spoon. If you're handy with the spat you might manage, but the cheese is hot and likely to have very hot oil + it is a really difficult mass to handle. Make sure you have enough cheese that a network of cheese strings will appear if you try to tear the mass and pieces apart from the rest with a spatula.

Don't scrape off the residue that does not easily come with, and if you are making another round with the same pan you have to make sure to scrape off any cheese bits, onion, meat etc as they will become extremely potent in the next baguette and are also likely to give off steam. So it's best to rather take a new clean pan, as also it will take really long time and a lot of effort cleaning that, that is why it is best to submerge it in water for at least half an hour before coarse pre-cleaning.

The baguette is done, but to make it more eating-friendly it is best to stuff the mass into the baguette. Use the bread knife and stuff it in carefully with the side.

If you don't care about the full authentic experience you can be a rebel toward the baguette gods and cut it in half too. Use the bread knife as if you are sawing and start going back and forth really carefully until it starts getting a grip on the bread and then starts gripping into there. Cut with very careful but decisive movement to not make the mass start falling out. In one way it is better because then you can start in the middle instead of having to bite your way to the good stuff.