First julian (slice long and thin) your onions and bell peppers and mince your cloves of garlic.

Sprinkle the mushrooms with salt and pepper on both sides so theyll have a little flavor

Put a large skillet over medium heat and drop your oil in so it can get nice and hot.

Take six hamburger buns and toast them so that they are nice and crunchy. Let them toast while your veggies cook as they will be done about the same time.

Once your oil reaches a nice temperature youll want to drop the sliced bell peppers, onions and garlic on. Sauté until the garlic gets a nice brown color on its edges and the onions become translucent.

Push your veggies off to the side of the skillet and drop your mushrooms and stock in. The veggies and mushrooms will soak in the stock giving it a nice taste. Cook till mushrooms are tender about 5-10 minutes.

By now your veggies and bread should be done. Take toasted buns and spread spicy mustard on both sides. This will give the sandwhich a nice kick.

Place portabello slices on top of bun first. Onions and bell peppers next. Finally top it off with a bit of spinach.

Eat and enjoy!