Step 1

Place tomatoes with their juices in a large bowl and crush with your hands. Heat 1 tablespoon oil in a large broiler-safe skillet over medium-high. Add onion; cook, stirring often, until golden, 6 to 8 minutes. Add oregano and ½ teaspoon kosher salt; cook, stirring constantly, until fragrant, about 30 seconds. Add crushed tomatoes; simmer, stirring occasionally, for 15 minutes.

Step 2

Add beans, 1 cup water, and remaining ½ teaspoon kosher salt to skillet. Simmer, stirring and mashing about half of beans, until sauce thickens slightly, 6 to 8 minutes. Meanwhile, preheat broiler with oven rack 6 inches from heat.

Step 3

Nestle polenta slices into sauce, overlapping slightly. Brush polenta with 1 tablespoon oil. Transfer skillet to oven; broil until bubbling and browned in spots, 4 to 6 minutes. Top with finely chopped parsley and flaky sea salt. Drizzle with remaining 1 tablespoon oil.