Summer Vegetable Spaghetti

2 C small yellow onions, cut in eighths
2 C (about 1 lb) ripe tomatoes, peeled, chopped
2 C (about 1 lb) yellow and green squash, thinly sliced
11/2 C (about 1/2 lb) fresh green beans, cut
2/3 C water
2 Tbsp fresh parsley, minced
1 clove garlic, minced
1/2 tsp chili powder
1/4 tsp salt
to taste black pepper
1 can (6 oz) tomato paste
1 lb spaghetti, uncooked
1/2 C Parmesan cheese, grated