It's easy to make an onion quiche. What isn't so easy is making a good one. Actually, it's damn hard. The great stumbling block is the pastry, known to the French as pate brisee. Tricky and temperamental, it can drive even good cooks to despair. Which hasn't deterred Cafe Europa (2536 Nottingham, 942-0001). The quiche here -- a good-sized slice costs $5.75 -- is exemplary. One reason is butter. It lends the pastry a pleasant nuttiness. And it helps as well that the crust is baked for 10 to 15 minutes before the eggs are added, which prevents a soggy bottom. And we all know how unpleasant that can be.