Preheat oven to 425 degrees. Toss shrimp in 1 tablespoon olive oil to coat and roast in preheated oven for 5-6 minutes or until almost cooked through. Set aside.
In a large oven-proof skillet ( I use a cast-iron skillet), cook garlic in 2 tablespoons olive oil over medium heat until lightly browned 2-3 minutes— stir frequently and lower heat to ensure the garlic doesn’t burn. Add anchovy paste (or anchovies), tomato paste, red pepper flakes. Cook, stirring until tomato paste darkens, about 2-3 minutes.
Add the tomatoes you have broken into pieces and continue to cook until liquid is absorbed about 8-10 minutes.
Add the rinsed and drained cannellini beans, the spinach and the reserved tomato liquid, salt & pepper to taste and simmer until heated through and the flavours have melded. ( Add 1/4 cup water if the mixture looks too dry).
Preheat broiler to high.
Arrange roasted shrimp on top of bean mixture scatter mozzarella cheese over the top. Broil until cheese is melted, bubbling and browned in spots, about 2 minutes (watch closely to ensure the cheese does not burn).
Serve immediately- right from the skillet, with Parmesan cheese for sprinkling and crusty bread for dipping.