Bring water to a boil in a Dutch oven; add kale. Cover and cook over medium heat 5 minutes or until tender. Remove kale with a slotted spoon, reserving cooking liquid. Chop kale and set aside.
Add potato to reserved cooking liquid in pan; bring to a boil. Reduce heat, and simmer 10 minutes or until tender. Drain; partially mash potatoes. Stir in kale and 1/4 teaspoon salt.
Preheat oven to 400°.
Heat oil and butter in a large nonstick skillet over medium-high heat. Add 1/2 teaspoon salt, diced onion, and chopped sage. Cook 13 minutes or until browned. Combine potato mixture, onion mixture, green onions, and pepper. Remove from heat; cool slightly. Divide potato mixture into 8 equal portions, shaping each into a 1/2-inch-thick patty. Place patties on a baking sheet coated with cooking spray. Bake at 400° for 20 minutes.
Broil patties for 5 minutes or until browned. Garnish with sage sprigs, if desired.