In a small heat-proof bowl, whisk together the Greek yogurt, one egg yolk, lemon juice, Dijon mustard, and cayenne pepper.
Place the bowl over a small pot of simmering water (double boiler method) and whisk constantly for 3-5 minutes until the sauce thickens into a velvety consistency; remove from heat and set aside.
Heat the ghee in a skillet over medium heat and sauté the fresh spinach until just wilted, then remove and set aside.
In the same skillet, sear the ham slices for 2 minutes per side until the edges are golden and crispy.
Split and toast the English muffin halves until they are lightly browned and crunchy.
Bring a separate pot of water to a gentle simmer with a splash of vinegar, crack the remaining two eggs into the water, and poach for 3 minutes until the whites are set but the yolks remain runny.
Assemble by placing the toasted muffin halves on a plate, layering with sautéed spinach, crispy ham, and a poached egg on each.
Generously drizzle the warm yogurt hollandaise over the eggs and season with sea salt and black pepper.