In a large pot, bring 4 quarts water to a boil. Add the pasta and 1 tablespoon salt, then cook until just tender but not fully cooked, about 2 minutes less than package directions. Drain the pasta, reserving 1½ cups of the cooking water.

Meanwhile, in a food processor, combine the onion, oil, ¼ cup water, garlic and ½ teaspoon salt and 1 teaspoon pepper. Add a third of the spinach and process until smooth, about 30 seconds. Add the remaining spinach in 2 batches, processing until smooth after each.

Transfer the spinach mixture to a 12-inch nonstick skillet over medium-high heat. Bring to a boil and cook, stirring occasionally, until it begins to thicken, 3 to 5 minutes.

Add the reserved pasta water and return to a simmer, then add the pasta and stir to coat. Simmer, stirring occasionally, until the pasta is al dente and the pesto no longer appears watery, 3 to 5 minutes. Stir in the heavy cream.

Off the heat, stir in the Parmesan, then taste and season with salt and pepper. Transfer to a serving dish, sprinkle with the queso fresco and serve with lime wedges.