Preheat the oven to 425°. Grease eight 6- to 8-oz. ramekins with the butter. Season with sea salt and freshly ground black pepper.
In a large skillet, heat the oil over medium. Add the sausage and bacon. Cook, stirring occasionally, breaking up the sausage with a spoon, until well browned, about 5 minutes. Add the onions; cook, stirring occasionally, until translucent, about 3 minutes. Add the spinach; season. Cook, stirring, until the spinach wilts, about 1 minute. Divide the spinach mixture among the ramekins.
Crack an egg into each ramekin; season. Place 2 tomato halves, cut side up, next to each egg.
Bake until the whites are set but the yolks are still runny, 8 to 12 minutes. Sprinkle with the parsley.