Fill a large pot with water and set it over high heat to boil.

Put a large skillet on the stove over medium heat and add the olive oil.

Add the chopped onion to the skillet. Cook, stirring now and then, until it softens and looks lightly golden at the edges, about 5 minutes.

Add the chopped garlic to the skillet. Cook for about 30 seconds, just until it smells fragrant, then turn off the heat.

When the water comes to a boil, add 1 1/2 teaspoons of the kosher salt to the pot. Add the penne and stir once so the pasta does not stick together.

Cook the pasta until it is just tender, using the package time as a guide.

While the pasta cooks, scrape the onion and garlic from the skillet into a blender.

Add the drained roasted red peppers, 4 ounces of the crumbled feta, half of the lemon zest, and 1 tablespoon of lemon juice to the blender.

Ladle 1/2 cup of the pasta cooking water into the blender. Blend until the sauce is smooth.

Pour the blended sauce back into the skillet and set the skillet over medium-low heat.

Cook the sauce in the skillet for 2 to 3 minutes, stirring a few times, until it is hot and slightly thicker.

Add the spinach to the skillet. Stir until the leaves wilt and are coated in the sauce, 1 to 2 minutes.