Melt butter in a 12-inch frying pan over medium heat. Once butter foams, add shallot and cook, stirring occasionally, until softened, about 5 minutes.

Add spinach, season well with salt and freshly ground black pepper, and cook until just wilted, about 3 minutes.

Make six indentations in spinach and crack an egg into each.

Cover the pan and cook eggs to desired doneness.

Meanwhile, combine crème fraîche or sour cream, lemon juice, and cayenne in a small bowl.

To serve, divide spinach and eggs among pieces of toast, place a dollop of cream mixture over each egg, and season with freshly ground black pepper.