When Mustard gets too tough to eat by sauteeing, we use this recipe:
2 bunches mustard greens
1/2 cup freshly grated Parmesan cheese
1/3 cup extra-virgin olive oil
1/4 cup lemon juice
3 large cloves garlic, minced
1 tablespoon soy sauce
1 minced anchovy fillet (optional)
1/2 teaspoon freshly ground pepper
1/4 teaspoon salt
Strip leaves from the stems (discard stems). Wash and dry the leaves. Tear the leaves into small pieces and place in a large bowl. Add Parmesan, oil, lemon juice, garlic, soy sauce, anchovy (if using), pepper and salt. With clean hands, firmly massage and crush the greens to work in the flavoring. Stop when the volume of greens is reduced by about half. The greens should look a little darker and somewhat shiny. Taste and adjust seasoning with more Parmesan, lemon juice, garlic, soy sauce and/or pepper, if desired.
Also, leave these in until spring. They'll reseed and next spring you should thin them so they do not overcrowd themselves. We have a patch of mustard that is on its 5th year, jusy by allowing them to reseed themselves.