Quick tomato puree:
1. In a saucepan, brown the onion. Pour in the crushed peeled tomatoes. Let simmer for 5 minutes and season. Pass through a sieve and cool.

For the Brussels waffles:

2. In a bowl, pour in the flour, sugar, 1 pinch of salt, and egg yolks. Mix and add the milk. Melt the butter and incorporate it into the mixture. Beat the egg whites with 1 pinch of salt until stiff peaks form. Gently incorporate the egg whites into the waffle batter. Cook the waffles without waiting, about 2 minutes on each side.