1. Preheat oven to 375°.
2. Place potatoes and 1 tablespoon salt in a large saucepan; cover with water. Bring to a boil. Reduce heat; simmer 15 minutes or until tender. Drain; cool slightly. Peel potatoes; discard peels. Press cooked potatoes through a ricer into a large bowl.
3. Heat oil in a large nonstick skillet over medium heat. Add garlic; cook 2 minutes, stirring frequently. Stir in romaine, spinach, and arugula; cook 1 minute or until greens wilt, tossing frequently. Remove greens from pan; finely chop.
4. Add greens, milk, fontina, eggs, and remaining 1/2 teaspoon salt to potatoes; stir well to combine. Coat a 9-inch pie plate with cooking spray; dust with breadcrumbs. Add potato mixture to prepared dish. Bake at 375° for 25 minutes. Remove from oven; sprinkle with pecorino. Let stand 10 minutes before serving.