Vegetable Burritos with Avocado Sauce

Excellent buffet fare. These burritos are filling enough for a luncheon or a light supper.

Burritos
1/4 cup vegetable oil
2 cloves garlic, minced
2 medium onions, chopped
5 (6 ounce) cans sliced water chestnuts, drained
6 large zucchini, shredded
1 1/2 pounds mushrooms, sliced
1 teaspoon celery salt
4 large tomatoes, chopped
Salt and pepper, to taste
16 large flour tortillas
4 cups shredded Cheddar cheese

In large skillet, heat oil over medium high heat. Add garlic and onion. Saut