Preheat oven to 400 degrees. Divide spinach among four 16-inch-long pieces of parchment paper.
In a small mixing bowl, toss asparagus, zucchini and summer squash with salt, pepper, garlic powder, and extra-virgin olive oil. Divide tossed veggies evenly among the four pieces of parchment paper with the spinach.
Pat salmon dry. Season with salt and pepper. Top with shallots, garlic, fresh herbs, and lemon slices.
Bring ends of paper together and fold down the edges around the salmon until a sealed packet has formed (watch the video above for steps!). Make sure to tuck ends underneath to secure.
Place salmon packets on a baking sheet and bake until packets are puffed and salmon is cooked through about 15-18 minutes.