Preheat oven to 350°F. Light coat a 9-by-13-inch baking dish with cooking spray.
Heat 1 tablespoon oil in a large skillet over medium heat until shimmering. Add chopped onion; cook, stirring occasionally, until softened and translucent, about 3 minutes. Add chopped sun-dried tomatoes and 1 tablespoon garlic; cook, stirring constantly, until fragrant, about 30 seconds. Add drained spinach; cook, stirring often, until nearly dry and bright green, 1 to 2 minutes. Remove from heat.
Transfer ½ cup of the spinach mixture to a medium bowl. Add softened cream cheese, ¾ cup feta, ¼ teaspoon pepper and the remaining 1 tablespoon oil; stir with a rubber spatula until smooth and evenly combined.
In a large bowl, whisk 12 eggs, 1 cup half-and-half, the remaining spinach mixture and ¼ teaspoon each salt, pepper and crushed red pepper and ¼ teaspoon black pepper until combined.
Pour the egg mixture into the prepared baking dish. Spread the cream cheese mixture over cut sides of 6 English muffin halves (about ¼ cup each); nestle them, cut-sides down, in the egg mixture, forming 2 rows of 3 muffins. Cover the dish tightly with foil.
Bake until the eggs are nearly set, about 25 minutes. Remove the foil and sprinkle the remaining ¼ cup feta between the muffins. Bake until the eggs are fully set in the middle, 15 to 18 minutes. Let rest for 10 minutes before slicing and serving.