Preheat oven 350ºF. Lightly grease, with olive oil or butter, four oven-safe ramekins or other oven single-serve baking dishes. Set aside.
Heat olive oil in a large pan over medium-high heat. Sauté onion and garlic, for a few minutes, then add chorizo, cook for about 8-10 minutes chorizo is completely cooked through, add tomato paste, stir until well incorporated, adding a little water, as needed, to get the paste moving, if necessary. Remove the pan from the heat and set aside.
Add about 1 1/2 cups of baby spinach to each of the four prepared ramekins. Press down and make a nest of sorts. Divide the chorizo mixture among the four dishes on top of the spinach.
Top each ramekin with 1-2 eggs. Place the ramekins into a large glass baking pan then add enough super hot water to that pan to got partly up the sides of the ramekins. Transfer the baking dish with the ramekins of eggs to the middle rack of the oven.
Bake for 15-25 minutes or until desired doneness. The whites should be cooked through but the yolk still soft.