Kale Pasta with Walnuts and Parmesan

walnut halves, roughly chopped

whole wheat spaghetti or other long pasta

extra virgin olive oil, plus additional for tossing with the pasta

large red onions, thinly sliced

large bunches kale (about 2 pounds), stems removed and chopped

garlic, minced

diced tomatoes in their juices

dry white wine, such as Pinot Grigio

crushed red pepper flakes (reduce to 1/4 teaspoon if sensitive to spice)

kosher salt, plus additional to taste

black pepper

freshly grated Parmesan cheese