Cheesy Buckwheat with Kale and Mushrooms

1 cup uncooked buckwheat 1 large egg, lightly beaten 1 1/2 cups water 1/2 teaspoon kosher salt 2 ounces shredded Gruyère cheese 2 tablespoons grated Parmesan cheese 1 tablespoon oil 1/2 cup diced onion 1 tablespoon sliced garlic 8 ounces presliced mushrooms 6 cups chopped kale 1/4 teaspoon black pepper 1/4 cup toasted walnuts 2 center-cut bacon slices, cooked and crumbled