Cook 3 sliced garlic cloves in olive oil in a large pot over medium-high heat until golden. Add 1 large head chopped escarole in batches, season with salt and red pepper flakes and cook until just wilted, 3 minutes. Add one 14-ounce can diced tomatoes; cook until the liquid is reduced, 7 minutes. Stir in 2 teaspoons chopped oregano; season with salt. Drizzle with olive oil and top with grated parmesan.