Whole Wheat Pasta Mixed w/ Arugula, Onions, & Tomatoes

**Another recipe from NieNie's 'CuisineNie'

Ingredients:

2 tablespoons olive oil
2 large onions, halved and thinly sliced (4 cups)
Coarse salt and ground pepper
12 ounces plum tomatoes, cored and diced (about 2 cups)
3/4 cup lentils, picked over and rinsed
12 ounces whole wheat pasta
1 bunch (8 ounces) arugula, stemmed and coarsely chopped
1/2 cup finely grated Parmesan cheese, plus more for serving (optional)

Directions:
Heat oil in a large skillet over medium-low heat. Add onions and 1/2 teaspoon salt; cover, and cook until onions wilt, about 20 minutes. Uncover; raise heat to medium. Cook, stirring often, until onions are dark brown, 20 to 25 minutes more.
Add 1/4 cup water; stir to loosen any browned bits from pan. Stir in tomatoes; remove from heat.
Meanwhile, in a medium saucepan, cover lentils with water by 1 inch. Bring to a simmer. Cover; cook until lentils are tender but still holding their shape, 15 to 20 minutes. Drain; stir into onion mixture. Season with salt and pepper.
Cook pasta in a pot of salted water until al dente. Reserve 1 cup pasta water; drain pasta, and return to pot.
Add lentil mixture, arugula, cheese, and reserved pasta water; toss. Season with salt and pepper. Serve with more cheese, if desired.