Thinly slice the onions and potatoes. Place them in a bowl, add 2 tablespoons olive oil, salt, and the semolina flour. Mix well.

Transfer the mixture to a baking sheet lined with parchment paper, gathering it toward the center.

Bake for 20 minutes at 350°F (180°C).

When the potatoes and onions are golden and crisp, adjust the salt if needed. Cover with the puff pastry and bake for another 15 minutes, until browned.

Flip the tart onto another sheet of parchment paper. Add your choice of cheese on top and return to the oven for another 10 minutes, until the cheese melts.

Continue baking in the oven, ideally a wood-fired oven if you have one.