In a large pot, bring salted water to a boil. Add pasta and cook for 8-9 minutes. With 5-6 minutes of cooking time left (7 for large asparagus spears), add asparagus pieces to the boiling pasta. With 1-2 minutes remaining, add peas. Reserve 1/2 cup pasta water.
Drain the pasta and veggies. While pasta is draining, add butter to the pot over medium heat. Once the butter is melted, add the shallot, red pepper flakes, and a pinch of salt; cook for 3 minutes.
Add garlic, cook for 1-2 minutes, then add 1/4 cup pasta water, stirring together to combine. Stir pasta and veggies back in; add more pasta water as needed for desired "sauciness". Salt and pepper to taste. Top pasta with a generous sprinkle of fresh parmesan and a squeeze of lemon.
Notes
To save on time, you can cook the sauce in a separate skillet and then add the pasta and veggies immediately after they're done cooking. I like to reduce the number of dishes and make it all in the same pot.
Parmesan: use vegetarian parmesan cheese, if needed.