Bring a large pot of salted water to a boil over high heat. Add the pasta and cook according to package directions until al dente. Add the asparagus to the boiling water the last few minutes of cooking time.
Place the frozen peas in the bottom of a colander, and drain the pasta and asparagus over the peas; set aside.
Meanwhile, melt the butter in a large skillet over medium-high heat. Add the garlic; sauté for 30-60 seconds, until fragrant.
In a small bowl, stir together vegetable broth and cornstarch until well blended; slowly whisk into the butter and garlic. Bring to a boil, stirring constantly, until the mixture begins to thicken.
Remove from the heat and stir in the cream, lemon juice, salt, pepper and red pepper flakes.
Add pasta and veggies to the cream sauce; gently toss to coat.
Garnish with lemon slices and season with more pepper, if desired. Serve immediately.