Position an oven rack about 5 inches underneath the broiler.
Heat the olive oil in a 12-inch or larger oven-proof skillet (see note) over medium-low heat. Add the onion and sauté until tender and translucent, about 4-5 minutes. Add the garlic and sauté until fragrant, 30-60 seconds.
Add the tomatoes with their juice, salt, pepper, oregano, and red pepper flakes. Bring to a boil, lower heat to medium-low and sauté uncovered, until the tomatoes are softened and some of the liquid has reduced, about 8 minutes.
Preheat the broiler to high. Arrange the shrimp over the tomato mixture in one layer. Simmer until the shrimp are ALMOST cooked. You want the shrimp to be barely pink on top. For medium shrimp, about 3 minutes. For large shrimp, about 5 minutes. For extra-large shrimp or chunks of fish, about 6-7 minutes. Sprinkle with feta cheese, if using, and pop under the broiler. Broil until the cheese is browned in some spots, about 1-2 minutes.
Remove from the oven and sprinkle with fresh herbs. Be very careful with the handle of the skillet since it will be hot. Cover with an oven mitt or a towel.