Preheat oven to 400°.

Bring a pan of water to a boil and cook jumbo shells for about 8 minutes, just until tender but not cooked all the way through. Drain and separate onto a sheet pan.

Chop the brisket into small pieces.

Add 2 cups of chopped brisket (reserve remaining), shredded colby-jack and green chiles to a bowl and mix to combine.

Stuff pasta shells with brisket mixture.

Make the sour cream sauce. Melt butter in a pan over medium heat, add flour and whisk to combine. Cook 2 minutes, whisking continually.

Whisk in chicken broth and milk.

Add sour cream, salt and garlic, whisking until combined. Continue to cook until thickened. Remove from heat.

Pour sauce into a casserole dish ( 8 x 11 or 3 1/2 quart).

Place stuffed shells into the sauce. Sprinkle reserved brisket over the shells.

Bake, uncovered for 30- 35 minutes or until sauce is bubbly and shells are tender.

Remove from oven. Garnish with diced tomatoes, crumbled queso fresco cheese and cilantro.

Serve with salsa and guacamole if desired.