Baked ziti but make it beans

This recipe answers the question "what if pasta was beans?" By that I mean it takes big white beans and treats them like pasta, tucking them into an herby tomato sauce with some fresh spinach, and topping them with copious amounts of cheese. A pared down version of Smitten Kitchen's pizza beans, this dish is easier and higher in protein than your typical pasta bake. It makes great leftovers and freezes beautifully. 

Cheesy Italian beans (baked ziti but make it beans)
1 T olive oil
1 medium onion, diced (optional)
1 heaping T minced garlic
15 oz can crushed tomatoes
3 (15-ounce) cans cannellini beans, drained (can substitute butter beans or great northern beans)
1 1/2 t Italian seasoning

3/4 t salt
black pepper
2 t red wine vinegar (optional, adds acidity to balance the earthiness of beans)
A few handfuls of fresh baby spinach (optional)

1/3 cup grated parmesan cheese 
1/3 pound grated mozzarella

Preheat oven to 450 degrees. In an ovenproof skillet or Dutch oven, heat the olive oil over medium-high heat. Sauté the onion, if using, over medium-high heat until golden, about 4-5 minutes. Add garlic and crushed tomatoes and sauté for another minute (this will caramelize the tomato a bit). Stir in remaining ingredients except for mozzarella. Sprinkle the mozzarella on top, and place pan in oven, uncovered, on top rack. The baby spinach will continue to wilt into the beans, and the parmesan will meld into the sauce as it bakes. Bake until the cheese melts and is bubbly, about 10 minutes. Serves 4-5.