Preheat oven to 375 degrees.
To make a crust:
In a food processor, mix cream cheese and margarine. Add salt and flour. Mix until a ball forms. Refrigerate. Roll out on a floured surface to fit an 8inch fluted tart pan. Weight with pie weights, beans or rice in foil, and bake for 15 minutes. Cool, remove from pan and set aside.
To make filling:
Plunge tomatoes into simmering water for 15 seconds. Drain. Remove skin and seeds. Place tomatoes in roasting pan with 1/2 teaspoon of the garlic, 1/3 cup of olive oil, and 1/4 teaspoon each of salt and pepper. Bake for 1 1/2 hours. Cool slightly and coarsely chop. In a small saucepan, bring vinegar to boil, and cook until reduced to 2 tablespoons. Add reduced vinegar to the tomato mixture and set aside.
In a large skillet, heat 2 tablespoons of the olive oil over medium heat. Add the remaining 2 teaspoons garlic and cook until golden. Remove and set aside.
In the same skillet, add the remaining tablespoon of olive oil. Increase heat and sear portobello strips on all sides until browned. Stir in the reserved garlic, turn off heat but keep warm.
Place spinach in the cooled tart shell. Arrange warm portobellos like spokes of a wheel on top of spinach. Sprinkle with salt and pepper. Top with mozzarella. (the tart may be assembled to this point and set aside.) Preheat the broiler. Broil briefly until cheese is melted. Drizzle each slice with tomato vinaigrette, garnish with capers and serve warm