Heat olive oil in a skillet over medium-high heat. Add the sliced yellow squash and diced green zucchini. Sauté for 5-7 minutes to evaporate excess water. Remove from pan.

In the same skillet, wilt the chopped spinach for 2-3 minutes. Transfer to a clean towel and vigorously wring out all excess liquid once cool enough to handle.

Melt butter in a saucepan over medium heat. Whisk in flour and cook for 1 minute. Gradually whisk in whole milk. Simmer until thick and creamy. Season with salt, garlic powder, and nutmeg.

In a bowl, combine the smooth ricotta cheese, egg, and remaining salt. Fold until completely mixed to create a stable, smooth cheese layer.

Boil the ruffled lasagna noodles in heavily salted water for 2 minutes less than package instructions until very al dente. Drain and lay flat on parchment paper.

Preheat oven to 400°F (200°C). In a rectangular white ceramic baking dish, layer béchamel, noodles, ricotta, veggies, béchamel drizzle, and mozzarella. Repeat layers, finishing with noodles on top.

Top with remaining béchamel and mozzarella. Cover tightly with foil and bake for 30 minutes.

Remove foil and bake 10-15 more minutes. Broil for the final 2 minutes until perfectly charred crispy spots form on the golden-brown mozzarella. Garnish heavily with fresh chopped parsley and coarse black pepper. Rest for 15 minutes before slicing.