Put a large pot of salted water on to boil for the tortellini.
Preheat the oven to 425°F and liberally coat a large, rimmed baking sheet with nonstick spray.
Trim the woody ends from the asparagus spears and peel away a bit of the outer skin toward the bottom to ensure tenderness and even cooking.
Cut each of the spears into 3 or 4 pieces and toss with a little salt and pepper, and a drizzle of olive oil.
Arrange the asparagus on the prepared baking sheet in a single layer and roast until tender, 8 to 10 minutes. (The exact timing will depend on the thickness of the spears.)
While the asparagus roasts, heat the butter and 1 tablespoon of olive oil in a small frying pan over medium heat.
Add the chopped onion and sauté for 3 to 4 minutes, until soft and translucent. Add the garlic and continue cooking until the mixture is pale golden in color, about 1 minute longer.
Deglaze the pan with the white wine, scraping up any browned bits that may have accumulated on the bottom.
Simmer for 1 to 2 minutes, then reduce the heat to low to keep warm.
Cook the tortellini until tender, then drain.
Add the asparagus to the garlic-onion mixture, combine and increase the heat to medium.
Stir in the lemon juice, zest, and cream. Add the tortellini and toss to combine.
Taste and adjust the seasoning if needed, then transfer to a serving dish.
Sprinkle with parsley, grind a little black pepper over the top and serve.
Tips for Making This Recipe
Buying and storing asparagus: Look for smooth stalks with tightly closed buds. If you're not going to cook it the same day, trim about half an inch from the bottom of the spears and place them upright in a container with some water in the bottom. Cover with a plastic bag and the asparagus should keep for up to three days (be sure to keep the water level up).
Cooking asparagus: Oven-roasting really brings out the natural sweetness of asparagus and we think it adds depth of flavor to this dish. Alternately, you could par-cook the asparagus in the microwave, then finish it in the pan with the garlic and onion.
How to get perfectly cooked tortellini: For best results when cooking any filled pasta, bring a pot of salted water to a rolling boil, add the pasta, then turn the heat down a notch. If you've ever had tortellini or ravioli break apart in the water, it's most likely because it was cooking too vigorously. A slight reduction in temperature will help with that.
How to achieve vibrant lemon flavor: When cooking with fresh citrus juice and zest of any type, try to add it to your other ingredients as close to serving time as you can. This will preserve its crisp acidity and fresh flavor.