Preheat oven to 450°. In a large bowl mix together bread crumbs and 1/2 cup of the milk and allow to sit for 5 minutes. Meanwhile, in a medium bowl whisk together sourdough starter, 1 cup milk, and 2 of the eggs until starter has dissolved. Add flour and a pinch of salt and whisk to make a lumpy batter. Set batter aside.

To the breadcrumbs and milk add ground beef, remaining egg, mozzarella, 1/2 tsp oregano, half of the garlic, 1/2 tsp salt, 1/4 tsp pepper and mix until combined. Form into 6 to 8 meatballs and place on a plate.

Heat 1 tbsp olive oil in a 12 inch oven-safe skillet over medium heat. Add meatballs and cook for 3-4 minutes per side, until browned. Remove from heat. Loosen meatballs from the bottom of the pan. Add 1 tbsp olive oil to the pan.

Spoon batter around meatballs to cover a bit more than half of each one (there may be extra batter). Gently lift meatballs with tongs to allow some batter underneath. Sprinkle rosemary over. Bake for 25-30 minutes until evenly browned.

Meanwhile, in a saucepan heat 1 tbsp olive oil over medium-low heat. Add the remaining garlic and cook, stirring frequently, for 1-2 minutes until fragrant. Add tomatoes, 1/2 tsp oregano, red pepper flakes, and a pinch of salt. Bring to a simmer then reduce heat to low and cook, stirring occasionally, for 10-12 minutes.

When toad-in-the-hole is done cut and serve immediately with the marinara sauce.