Preheat oven to 425° F with rack in middle.
Butter a 3-quart baking dish (about 13 by 9 by 2
tomatoes reserving 1 cup juice, then chop.
tablespoons butter in a 5-to-6-quart heavy pot over
medium-high heat until foam subsides, and then cook onions
with ½ teaspoon salt stirring occasionally, until
golden-brown, about 10 minutes.
garlic, allspice, and cloves and cook, stirring, 1 minute.
Stir in tomatoes with reserved juice, thyme, brown sugar,
and ½ teaspoon pepper and briskly simmer, uncovered,
stirring occasionally, until thickened, about 20 minutes.
Meanwhile, melt remaining ½ stick butter and brush onto both
sides of bread.
Have each slice.
Transfer tomato mixture to baking dish and top with bread,
Bake until bubbling and bread is crisp and golden-brown,
about 20 minutes.