Preheat the broiler. Arrange the tomatillos, jalapeño, onion, and garlic on a sheet pan and broil for 8 to 10 minutes, turning once, until blistered and charred in spots.
Let everything cool slightly, then peel the garlic. Transfer to a blender with the cilantro, lime juice, and 1/4 cup of the stock. Blend until mostly smooth.
Heat the olive oil in a large, deep skillet over medium heat. Pour in the blended salsa carefully, then stir in the cumin, salt, and remaining stock.
Simmer uncovered for 10 to 12 minutes, until the sauce thickens and deepens to a solid, dark green.
Stir in the spinach and cook for about 1 minute, just until wilted.
Use the back of a spoon to make 6 small wells in the sauce, spaced apart. Crack an egg into each well.
Cover the skillet, reduce heat to low, and cook for 6 to 8 minutes, until whites are set but yolks still jiggle when the pan is shaken gently.
Remove from heat and top with queso fresco, avocado, and extra cilantro. Serve with warm tortillas.