Preheat oven to 425°F (220°C). On a rimmed baking sheet, toss asparagus with olive oil, salt, and pepper. Arrange in a single layer and roast for 12-15 minutes, until tender and lightly charred at the tips. Remove and set aside.
Bring a large pot of water to a rolling boil. Add 1 tablespoon salt. Drop in the fresh pasta and cook until just al dente, about 2-3 minutes. Reserve 1 cup of the starchy pasta water, then drain the pasta (do not rinse).
While the asparagus roasts and water boils, melt butter in a large skillet over medium heat. Add minced garlic and cook until fragrant, about 1 minute (do not brown). Pour in the heavy cream, lemon zest, and lemon juice. Whisk to combine and bring to a gentle simmer. Cook for 2-3 minutes, stirring occasionally, until the sauce thickens slightly. Reduce heat to low, then whisk in the grated Parmesan until smooth. Season with salt, pepper, and red pepper flakes if using.
Add the drained fresh pasta directly into the skillet with the cream sauce. Toss gently with tongs, adding reserved pasta water a splash at a time, until the sauce coats each strand and is silky. Turn off the heat.
Divide the pasta among plates or shallow bowls. Arrange roasted asparagus spears on top or alongside. Finish with a shower of extra Parmesan, fresh herbs if using, and an extra crack of black pepper. Serve immediately.