Heat skillet on stove top on high heat with a drizzle of olive oil.
Place roast in pan and sear on both sides.
Place in slow cooker on high heat.
Slice onion and place on top of roast with garlic, the two packets of fajita seasoning and 1/2 cup water. Let cook for 4 hours or until you can shred the beef with a fork.
At that point, shred the beef. I find using two forks work the best. Add the green chili and the chopped cilantro and mix together.
Preheat oven to 400*
Heat refried beans on the stove til they are heated and creamy and easily spreadable.
Warm soft taco shells in microwave (about 30 sec, just until warm so the can be rolled)
Spray baking sheets with cooking spray
Assemble chimis by spreading beans, cheese, beef mixture on tortilla shells then fold. I fold them by folding the shell by a third, tucking in the sides and then folding them the rest of the way over. Eyeball how much of each to put on one shell, depending on how big your shells are. Place on baking sheet seam side down. Using a pastry brush, brush the finished chimis with vegetable oil if you are using that or spray them with cooking spray if that is what you are using.
Place them in oven for about 15 minutes. Keep an eye on the bottom of the chimis, you don't want them to burn (depends on what kind of baking sheet you are using)
Take the chimis out of oven.
Turn on the oven broiler.
Top chimis with cheese and place them under the broiler for 2-5 minutes til cheese is bubbly. (keep a close watch!! You don't want them to burn!)