Spinach and Onion Pizza

refrigerated fresh pizza dough

canola oil, divided

vertically sliced onion (about 1/2 large)

garlic cloves, thinly sliced

no-salt-added whole tomatoes, well drained and coarsely chopped (or tomatoes reserved from Braised Pork with Slow-Cooked Collards, Grits, and Tomato Gravy)

chopped fresh oregano, divided

crushed red pepper

kosher salt

fresh mozzarella cheese, torn into bite-sized pieces

Parmesan cheese, grated (about 1/3 cup)

fresh baby spinach