Kale and other dark, leafy greens are very high in calcium. If kale isn't your favorite, try turnip, beet, or mustard greens. You can also substitute spinach in this recipe but skip the first (boiling) step.
1-1 1/2 pounds kale, ribs removed, washed, coarsely chopped
1 tablespoon extra-virgin olive oil
1 small onion, sliced
1 teaspoon minced garlic
1/2 teaspoon kosher salt
1/4 teaspoon freshly cracked black pepper
1 1/2 tablespoons balsamic vinegar
Bring a large pot of salted water to a boil. Add kale and cook uncovered, stirring occasionally, until tender, 10 to 15 minutes. Drain and set aside.
Heat olive oil in a Dutch oven over medium heat. Add onion and cook until tender, 4 to 5 minutes. Add garlic and cook 30 seconds. Add kale, salt, and pepper and cook, tossing to combine, until thoroughly heated.
Remove from heat and stir in vinegar. Serve immediately.