My quiche turned out great! 8/10, it did feel like a little win. The main issue was too much filling making it feel too heavy. I'm going to share the fixed recipe here:

Ricotta and spinach quiche, serves 2 with leftovers

a 30cm large disc of puff pastry
veg oil
1/4 medium onion, diced
100g fresh spinach, shredded
2 eggs
2 Tbsp milk
salt, pepper, nutmeg to taste (seasoning)
(30g? eyeballed) Parmesan cheese to taste, grated
120g ricotta, crushed
a dozen cherry tomatoes

Shape the pastry inside a cake or pie pan. Make sure that there's enough pastry to get a nice border. Leave it in the fridge for now.
In another pan, cook the diced onion with a bit of veg oil, until translucent. Add spinach and quickly cook it until wilted; then let it cool enough so the eggs don't get cooked in the next step.
Whisk together eggs, milk, seasoning; add them to the pan. Then add the crushed ricotta and some (not all) Parmesan cheese. Mix everything well.
Spread the mix over the puffed pastry. Sprinkle the rest of the Parmesan cheese over it, and plop the cherry tomatoes.
Gently fold inwards the border of the puffed pastry, over the filling. It's optional, I just think that it looks nicer this way.
Bake it in a [important!] pre-heated oven at 180°C, until the filling is set, the top is golden, and the cherry tomatoes are slightly wilted. This should take half a hour or so. Let it rest and serve.