Sprinkle 1/2 sliced lemon with 1 teaspoon kosher salt and set aside. Cook 12 ounces green beans (trimmed) in boiling salted water until crisp-tender, 5 minutes; drain and rinse. Saute 2 smashed garlic cloves in olive oil over medium heat, 3 minutes. Add one 15-ounce can cannellini beans (drained and rinsed), the green beans and a pinch of red pepper flakes; cook 2 minutes. Pat the lemon slices dry; chop and add to the beans along with some chopped parsley. Season with salt.