Choose a 10- to 12-inch saute pan with a lid. Put in the butter and onion, and turn on the heat to medium-high. Cook, stirring well, for about 1 minute, then cover the pan and turn to low. The onion must cook until it's tender and a very pale gold color, but not brown.

Drop the pasta into a large pot of boiling, salted water. The moment it is barely tender, but still firm to the bite, remove from stove and drain.

When the onion is done, uncover the pan, add the tomatoes, and turn up the heat to high. Cook, stirring frequently, for 6 to 8 minutes. Add salt and very generous grindings of pepper. When done, the tomatoes should be soft, but not dissolved into the sauce.

Toss the pasta with the sauce in a warm serving bowl. Add grated cheese, toss again and serve at once. Serves 4 to 6.