Brush eggplant and red bell peppers with 1 tablespoon olive oil; use more if necessary, depending on sizes of vegetables. Place on a baking sheet and roast in preheated oven. Roast eggplant until tender, about 25 minutes; roast peppers until blackened. Remove from oven and set aside to cool.
Meanwhile, heat 1 tablespoon olive oil and cook and stir mushrooms until tender. Stir crushed garlic into mayonnaise. Slice focaccia in half lengthwise. Spread mayonnaise mixture on one or both halves.
Peel cooled peppers, core, and slice. Arrange eggplant, peppers and mushrooms on focaccia. Wrap sandwich in plastic wrap; place a cutting board on top of it and weight it down with some canned foods. Allow sandwich to sit for 2 hours before slicing and serving.