Slow Cooker Vegetable Spaghetti Sauce

1 large onion, chopped (1 cup)

2 medium carrots, chopped (1 cup)

2 cups sliced fresh mushrooms (6 oz)

1 medium green bell pepper, chopped (1 cup)

2 cans (14 1/2 oz each) Muir Glen® organic diced tomatoes with Italian herbs, undrained

2 cans (8 oz each) Muir Glen® organic tomato sauce

1 can (6 oz) Italian-style tomato paste

2 teaspoons sugar

1/2 teaspoon salt

1/2 teaspoon fennel seed, crushed, if desired

12 oz uncooked spaghetti

Grated Parmesan cheese, if desired