Cut bread into 1/2-inch chunks. In a food processor or blender, whirl into coarse crumbs; you should have 2/3 cup.
Rinse zucchini and trim and discard ends. Cut into 1/8-inch-thick rounds.
In a 5- to 6-quart pan over medium heat, stir bread crumbs often until golden and crisp, about 5 minutes. Pour into a bowl.
Add 3 tablespoons olive oil to pan and increase heat to medium-high. Stir in zucchini, garlic, chopped oregano, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Cover and cook, stirring occasionally, until zucchini begins to soften, about 5 minutes. Uncover and stir often until zucchini is browned and soft when pressed, 7 to 10 minutes longer. Remove from heat and stir in toasted crumbs, the parmesan cheese, and more salt and pepper to taste.
Generously oil six to eight ramekins (1/2- to 3/4-cup size) with 1 tablespoon olive oil. Divide zucchini mixture evenly among ramekins and press in firmly. Let stand at room temperature at least 5 minutes or up to 4 hours (see notes).
Rinse and core tomatoes; cut crosswise into 1/4-inch-thick slices. Arrange 3 tomato slices on each of 6 to 8 salad or dinner plates. Run a knife around inside edge of ramekins and invert zucchini timbales onto tomatoes; if necessary, hold each ramekin and plate together and shake gently to release. If desired, garnish with oregano sprigs. Add more salt and pepper to taste. If desired, drizzle a little olive oil over tomatoes to taste.