Easy Grilled Cheese Sandwich with Mushrooms and Kale

Name: Easy Grilled Cheese Sandwich with Mushrooms and Kale
Ingredients: 2 tablespoon olive oil; 4 medium portobello mushrooms (caps and gills removed and then sliced); 1 cup diced onion; 2 clove garlic (peeled and chopped); 4 cup loosely packed, chopped black kale leaves (or other sturdy green such as collards); ½ cup water; salt and pepper (as needed for seasoning); 8 slice good rye bread; 4 tablespoon grainy mustard (or to taste); 8 sandwich sized thin slices Gruyere cheese; 8 tablespoon unsalted butter