Heat the olive oil in a large skillet or Dutch oven over medium heat. Then, add the garlic, onions, and asparagus; sauté until the ingredients are fragrant and tender—about 5-7 minutes. Once these ingredients are tender, carefully transfer them to a bowl with a slotted spoon and cover the bowl with a plate to trap the heat. Set them aside.
Pour two cups of the vegetable broth into that same cooking vessel and bring the broth to a simmer. Scrape any browned bits to deglaze the pan. This will add extra flavor. Add the pasta, fully immersing it (or as much as you can) into the simmering broth, and lower the heat to medium-low. Add the fresh squeezed lemon juice.
Let the noodles simmer in the broth until they're al dente or your preference of tenderness. In general, this should take about 25 minutes. Add extra broth as needed throughout the cooking process. If you're out of broth, simply use water. The noodles soak up a lot of the liquid as they cook, so you want to keep the cooking vessel replenished with liquid until the noodles are fully cooked, otherwise you'll be frying them.
When the noodles are near done, add the milk and the cream cheese, as well as a pinch of kosher salt and cracked black pepper. Stir well to mix and melt the cheese, and keep tossing the pasta until completely coated. Once the sauce has thickened and coated the pasta, add the cooked onion/garlic/asparagus you set aside earlier to the noodles as well as the spinach, capers, lemon zest, and smoked salmon. Toss again to distribute the ingredients throughout the pasta.
Remove the pasta from the heat, cover it, and let it stand for five minutes. Season the pasta with extra salt and pepper if needed.
Serve with a lemon wedge for an extra spritz of lemon, if you'd like!