We were so beguiled by the sandwiches at this tiny (10-table), cash-only, old-style North Beach restaurant that it took an idle comment by a friend to shake us from our habits. He said that his favorite pizza by the slice was the porcini sold here. We took his advice, only to find that the porcini (saut?ed imported Italian mushrooms, made bianca-style, i.e., without tomato sauce) is our second favorite of the nine combinations on offer. We love L'Osteria's amazingly thin, amazingly crisp crust, and we love it best with the tre formaggi (Gorgonzola, ricotta, mozzarella) topping, like an especially toothsome grilled cheese sandwich. Coming up a close third is the salsiccia: mushrooms, morsels of Italian sausage, mozzarella, tomato sauce. But the different variations are all good, because of the good crust, excellent ingredients, and care used in their creation. You're welcome to sit at a table and enjoy your slice, which is also one of North Beach's true bargains, at between $2.50 (for the margherita, mozzarella, and tomato sauce) and $4 (tre formaggi, porcini) a slice.