Preheat oven to 350°. In a large skillet, cook sausage, mushrooms, onion and peppers over medium heat until meat is no longer pink and vegetables are tender; drain. Return sausage mixture to skillet. Whisk five eggs; add to skillet. Cook and stir over medium-high heat until set; stir in Havarti cheese. Set aside.
In a small bowl, combine the spinach, ricotta cheese, Parmesan cheese, basil and remaining egg. Brush a 13x9-in. baking pan with butter. Unroll phyllo dough; trim to fit into pan.
Layer 10 sheets in prepared pan, brushing each sheet with butter. (Keep remaining phyllo covered with plastic wrap and a damp towel to prevent it from drying out.) Top with half of the sausage mixture.
Layer with 10 additional phyllo sheets, brushing each with butter; spread with ricotta mixture. Layer with five phyllo sheets, brushing again with butter; top with remaining sausage mixture. Layer with remaining phyllo and butter. Using a sharp knife, cut into 12 rectangles. Bake, uncovered, 30-40 minutes or until golden brown.
Freeze option: Cover and freeze unbaked casserole for up to 3 months. To use, thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Bake, uncovered, at 350° for 40-50 minutes or until golden brown and heated through.
Editor's Note: Look for phyllo dough in the frozen pastry section.