Heat the oven to 400°. Arrange the phyllo cups on a rimmed baking sheet. In a small bowl, lightly whisk together an egg, a pinch each of oregano and garlic powder, and pepper to taste. Add 1 cup finely chopped baby spinach and 3 tablespoons crumbled feta and stir to combine. Evenly distribute the mixture among the cups, then bake them until they're lightly browned and puffed, about 12 minutes.