(good picnic food, too)
This won't be a very long post, partly because plenty of people are already aware of Mamma Roma's six branches in Brussels (there are also three in Paris, apparently), but its pizza by the slice is certainly a little different from the standard selection: what is available seems to vary, but although I have tried a fairly substantial pepperoni-based offering, and the website mentions other favourites like Capricciosa and 4 Formaggi, these are in a minority. On my two visits (several months apart!) I have enjoyed interesting combinations that included courgettes and Fontina cheese, broccoli, spicy salsiccia sausage and buffalo mozzarella, and one involving cooked ham and mozzarella but also anchovy sauce of a disconcertingly grey hue (reheating toned down its strong flavour and turned it into a subtle counter to the richness of the cheese).
I assume that you can have your slices cold, but most people ask for theirs to be reheated by staff who have mysterious ways of knowing which ones belong to which customer and when they need to come out of the oven. This is, of course, not the place to come for a healthy meal - the 'traybaked' nature of the pizzas is such that each slice constitutes a hefty piece of thick dough (though the crispiness of the underside and the slight moistness of the top are just right) and, while the toppings, are fairly generous I feel it's the base that takes centre stage. Still, if you don't mind falling asleep in the afternoon, the 8.90E lunch deal for two big slices (otherwise sold by weight) and a drink is pretty good value. I have no idea if this is Brussels' best pizza, but it will certainly do for the moment.
37 Place Jourdan
(for other locations see the website)