In a stockpot, bring water and 2 teaspoons salt to a boil. When boiling, add kale, return to a boil, stir, cover and reduce heat to low. Boil gently for about 20 minutes, stirring several times to make sure greens are submerged. Remove from heat and drain in a colander, then return greens to stockpot, covered but off heat.
While greens are cooking, make sauce. In a medium saucepan over medium heat, melt butter and cook onions for about 10 minutes, stirring occasionally, until they just begin to brown. Add paprika, rosemary, nutmeg, salt and black pepper. Cook and stir for 30-60 seconds. Add flour and cook over medium-low heat for about 1 minute, stirring frequently; it will look pasty. Add milk, stirring well to incorporate; it will appear curdled at first, but will rapidly smooth out. Bring to a boil, stir and boil gently for 1 or so minutes, until thickened. Add sauce to pot with drained greens and cook and stir to heat through. Taste and adjust seasonings as needed.