Preheat oven to 350 degrees F. Spray a 9x13 inch baking dish with nonstick cooking spray and set aside.
In a large skillet, heat olive oil over high heat. Add the onion and cook until tender, about 3 minutes. Stir in the asparagus, broccoli, spinach, and garlic. Cook for 4 minutes, or until vegetables are tender and spinach is wilted. Stir in the chopped artichoke hearts, basil, and half of the green onions. Season with salt and black pepper, to taste. Pour vegetable mixture into prepared pan and spread out evenly. Sprinkle the Havarti cheese evenly over the vegetables.
In a medium bowl, combine eggs and milk. Whisk well. Season with salt and black pepper, to taste. Pour egg mixture evenly over the veggies and cheese. Top with remaining green onions and crumbled feta cheese.
Bake for 30-35 minutes or until eggs are set and slightly golden around the edges. A knife inserted into the center should come out clean. Remove from oven and let cool for 10 minutes. Cut into squares and serve warm.
Note-this casserole reheats well and can be made a day in advance. You can omit the cheese if you are dairy-free!
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