Trim and peel <quantity>1<unit>kg</unit></quantity> of red onions

Cut the onions in half and remove the stems before thinly slicing them

Mince <quantity>1<unit>tbsp</unit></quantity> of thyme

Mince <quantity>1<unit>tbsp</unit></quantity> of picked rosemary

Add the thyme and rosemary to the red onions in a bowl

Add <quantity>½<unit>cup</unit></quantity> of muscovado sugar

Season with salt and espelette chili powder to taste

Coat with olive oil to taste

Squeeze the juice of <quantity>½</quantity> lemon

Mix by hand, using a bit of force, until the onions begin to soften

Let the onions rest for 2h to marinate

Place <quantity>300<unit>g</unit></quantity> of puff pastry dough on a work surface and dust it with all-purpose flour

Press the puff pastry dough with a rolling pin to slightly flatten it

Roll out the dough until you obtain a <quantity>5<unit>mm</unit></quantity> thick rectangle

Place the dough on a silicone baking sheet. Place it in the refrigerator for 30m until cold

Preheat the oven to <temperature>180<unit>°C</unit></temperature>

Crack <quantity>3</quantity> eggs and separate the yolks in a bowl

Add <quantity>1<unit>tbsp</unit></quantity> of dijon mustard

Add a splash of water and mix until smooth

Remove the puff pastry dough from the refrigerator. Cut the edges to get a rectangle

Brush the dough with the yolk and mustard mixture

Spread the marinated red onions over the dough, leaving around <quantity>2<unit>cm</unit></quantity> on the edges

Season with salt and espelette chili powder to taste

Bake the tart for 30m until deep golden brown. The red onion tarte fine is ready

Enjoy!