Grill chiles until blistered on all sides. Place in airtight container until cool. Remove skin and seeds, cut into 1" strips. Combine with other filling ingredients. Set aside.
In large saucepan, soften onion in butter over medium heat, 4 min, without browning. Sprinkle in flour and cook 30 sec. more. Whisk in half and half and broth, bringing to a simmer, cooking 3-4 min until smooth. Remove from heat and add salt, pepper, enchilada sauce and sour cream (add a little more half and half if too thick). Set aside.
Dip tortillas into hot oil, shaking off excess, and immediately roll 1/2 c of filling into each one. Place in baking dish, top with sauce, and cover tightly with parchment paper and foil. Bake at 350 degrees until internal temp reaches 165 degrees, approx. 30 min. Remove cover, top with pepper jack and bake 10 min more. Top with olives, diced tomatoes, green onions and cilantro. Serve immediately.
*Hatch chiles are available in your Market Produce Department for a limited time each summer. If not available, substitute with Poblano, Anaheim or 4 oz canned hot Hatch green chiles and 4 oz mild Hatch chiles, or as desired.