Cut the tops off the zucchinis and hollow them out with a small spoon, leaving about a 1 cm wall. Set the flesh aside.
Peel and finely chop the onion, then sauté it in a bit of oil.
Add the zucchini flesh and cook for a few minutes until it releases its water.
Set aside two slices of Brie and cut the rest into small pieces.
Add these pieces of Brie to your mixture and gently melt them.
Stuff the zucchinis with this mixture, then place the reserved slices of Brie on top.
Arrange the zucchinis in an ovenproof dish, placing the tops (lids) next to them.
Bake for 30 minutes at 200°C (about 400°F).
Serve the stuffed zucchinis with their little caps, alongside a green salad.