In a large nonstick skillet, cook the sausage over medium heat, breaking it up with a wooden spoon, until crumbly and crisp, about 10 minutes. Remove the sausage to a bowl with a slotted spoon and return the skillet to the stove.
Add the olive oil, zest and juice to the pan and, over medium-high heat, stir the mixture with a wooden spoon, scraping up any brown bits, for about 1 minute. Remove the pan from the heat. Add the spinach and use tongs to coat the leaves with the dressing. Sprinkle the salad generously with salt and pepper. Divide the spinach evenly among 4 bowls and top with the crumbled sausage.
Wipe the nonstick skillet clean with a paper towel and fry each egg over easy. Top each salad with a fried egg and serve.