Preheat oven to 450F. Heat oil in large heavy skillet over medium flame. Cook onion,stirring occasionally, until soft. Meanwhile, halve the zucchini lengthwise and slice into 1/4-inch thick half-moons. When onion is soft add zucchini and garlic to the pan and cook, stirring occasionally, until zucchini is just tender. Add tomatoes and thyme and cook about 2 minutes more. Add wine or broth, and salt and pepper to taste and simmer until liquid is reduced by half, about 5 minutes. Remove skillet from heat and allow mixture to cool. Drain any accumulated moisture. Arrange bread slices on baking sheet and toast in preheated oven until very lightly colored, about 4 minutes. Mound about 2 tablespoons of zucchini mixture on each toast and sprinkle with a bit of the cheese. Return to oven until cheese is melted and beginning to brown a bit.