For the sauce: Combine the mayonnaise, mustard, 1 tablespoon warm water, lemon juice, and cayenne pepper in a blender and process until combined. Scrape out and set aside. Note: if you prefer more sauce on your eggs, double this up, we do!
For the eggs: Fill a wide pot with about 3 inches of water, add the vinegar and bring to a low simmer.
For Prosciutto: Preheat oven or toaster oven to 375 degrees.
Line rimmed baking sheet with parchment paper. Place prosciutto on a baking sheet, laying slices flat. Do NOT overlap pieces. Bake until the fat turns golden and meat is darker about 12 minutes. Using tongs, carefully transfer prosciutto to paper towels to drain (it will crisp as it cools).
Meanwhile, heat the oil in a large nonstick skillet. Add the shallots and cook, stirring frequently, until golden, about 1 minute. Add the spinach, remove from the heat, and stir until the spinach wilts, about 2 minutes. Season with pepper. Set aside and keep warm.
Toast the English Muffins in a toaster oven until they are lightly golden about 5 mins. Top each with a tomato slice and toast until the tomato is slightly warm, about 1 min. Top each tomato slice with the bacon or prosciutto and spinach mixture.
To poach the eggs crack each into a separate mug or bowl, and slip them into the now simmering water. Cook until the whites are at the desired doneness, 3 to 5 mins. Remove with a slotted spoon and place on each English muffin. Drizzle with the hollandaise.