Unroll crescent rolls; press dough on bottom and up sides of a 10-inch tart pan, pressing to seal perforations.
Bake at 375° for 6 minutes or until lightly browned. Gently press crust down with a wooden spoon. Spread crust with mustard, and set aside.
Melt butter in a large skillet over medium-high heat. Add squash, onion, and garlic; sauté 7 minutes or until tender. Remove from heat; stir in parsley and next 5 ingredients.
Whisk together eggs and milk in a large bowl; stir in cheese and vegetable mixture. Pour over crust.
Bake at 375° for 20 to 25 minutes or until a knife inserted in center comes out clean. Garnish, if desired.
*1 1/2 pounds zucchini may be substituted for yellow squash.