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1
Preheat the oven to 350 degrees F.
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2
Heat the olive oil in a large saucepan over medium heat.
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3
Add the onion and garlic and cook, stirring, until softened, about 4 minutes.
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4
Add the yellow squash and zucchini and cook, stirring, for 4 to 5 minutes more.
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5
Season with salt, pepper and red pepper flakes.
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6
Stir in the crushed tomatoes and tomato sauce; bring to a simmer.
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7
Cook, stirring occasionally, about 20 minutes.
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8
Add salt, pepper and a pinch of sugar, if necessary.
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9
In a large bowl, mix together the ricotta cheese, spinach, pesto and season with pepper.
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10
Grease the bottom and sides of your lasagna dish with butter.
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11
Spread a little of the tomato sauce on the bottom of the pan.
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12
Lay 4 noodles across the bottom of the baking dish, overlapping by 1/2-inch.
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13
Spread 1/3 of the ricotta and pesto mixture evenly on top.
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14
Sprinkle with about a 1/2 cup mozzarella cheese.
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15
Ladle about 1 cup of sauce on top.
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16
Repeat layering in the same order.
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17
Layer the top layer of noodles and drizzle with the remaining sauce, then sprinkle with the Parmesan and remaining mozzarella.
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18
Cover with foil and bake in the center of the oven until the sauce is bubbling around the edges, about 40 minutes.
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19
Uncover and bake 5 minutes more.
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20
Let stand 10 minutes before slicing.
