Preheat the oven to 375˚F. Butter six 8-ounce ramekins. Set aside.
1. Fill a 4- to 5-quart pot about three-quarters full of water and add 1 tablespoon of the salt. Bring to a boil and add the pasta. Cook, stirring once or twice, until tender but firm, about 4 minutes, and drain. Reserve the pot.
2. Place the English muffins cut side up on a rimmed baking sheet and spread them with 2 tablespoons of the butter. Toast in the oven until light brown around the edges, about 10 minutes. Remove from the oven and transfer the muffins to a plate. Reserve the baking sheet.
3. Using the same pot, you used to cook the pasta, melt the remaining 3 tablespoons butter over medium heat until browned. Watch careful and be careful not to burn it. Slowly whisk in the flour and stir constantly until a paste forms, 30 to 45 seconds. Continue stirring for about 2 minutes more, until the mixture starts to darken slightly and smell a bit nutty.
Slowly whisk in the milk, 1½ cups of the cream, and the remaining 1 teaspoon salt and cook until the mixture starts to thicken and is just beginning to bubble around the edges, 5-7 minutes. It should be thick enough to coat the back of a wooden spoon. Add the cheese, mustard powder, and cayenne and stir until the cheese is melted and the sauce is smooth and not too runny. It should be similar in texture to cake batter. If it’s soupy, continue cooking until it thickens.
4. Add the pasta and spinach and stir to combine. Fill the ramekins with the mixture to within 1 inch of the rim. Place on the reserved baking sheet and bake for 15 minutes.
5. Remove from the oven and top each with an English muffin, cut side up, followed by a slice of bacon. Break 1 egg into a small bowl or shallow baking dish. Carefully pour the egg on top of the bacon. Repeat with the remaining eggs.
Drizzle each ramekin with the remaining cream and sprinkle with salt and black pepper. Cook for an additional 10 minutes more, or until the egg white has turned solid white. Let cool for 10 minutes and then sprinkle each with a few drops of lemon juice just before serving.
NOTE: To make this in a baking dish, butter a 9 by 13-inch baking dish or pan. Use the same quantity of ingredients as specified. You don’t need to double it despite the use of the larger pan. Proceed as directed.