Slow Cooker Spaghetti Sauce

1 package (8 oz) sliced fresh mushrooms (3 cups)

2 large onions, chopped (2 cups)

3 cloves garlic, finely chopped

2 cans (15 oz each) Muir Glen® organic tomato sauce

1 can (28 oz) Muir Glen® organic diced tomatoes, undrained

1 can (6 oz) Muir Glen® organic tomato paste

2 tablespoons sugar

1 tablespoon dried basil leaves

1 teaspoon dried oregano leaves

1/2 teaspoon salt

1/2 teaspoon pepper

1/2 teaspoon crushed red pepper flakes