Preheat oven to 375°F. Cut tomatoes in half and spoon out flesh. Put flesh in a sieve set over a bowl and press down to release juices; reserve. Season inside of tomatoes with salt and pepper; invert over a baking sheet lined with a kitchen towel.
Melt butter with 2 Tbsp. oil in a skillet over low heat. Add onion and sauté for 5 minutes. Add garlic; sauté for 3 minutes. Remove from heat. Stir in bread crumbs, herbs and Parmesan. Stir in 1/4 cup of tomato juices, then egg.
Stuff tomatoes with breadcrumb mixture. Grease a 9-by-13-inch baking dish. Place tomatoes in dish in a single layer. Drizzle with 2 Tbsp. oil. Bake for 30 minutes, until topping is browned and crisp.