Preheat oven to broil. Place jalapeno peppers on a small baking sheet and place in oven. Broil, turning a few times during cooking, until peppers are starting to brown/char all around. This will take about 5 minutes. Remove from oven. Once cooled, deseed and roughly chop. Set aside.
Lower heat to 375 degrees F. In a medium saucepan over medium heat, heat a small amount of olive oil, enough to coat the bottom of a pan. Add onions and cook, stirring, until beginning to soften, about 6 minutes. Add in garlic, and continue to cook until garlic and onions are soft. Remove from heat. Add in spinach, stirring, and let wilt. Let mixture cool slightly.
In a large bowl, add onion/spinach mixture. Stir in chicken and 1 cup shredded Monterey jack cheese. Set aside.
In the same saucepan you used for the onions, combine cream cheese, sour cream, and half and half over low-medium heat. Cook, stirring, until cream cheese is melted and mixture is well combined. Add roasted jalapeños, and puree sauce using either an immersion or regular blender. Add about ½ of the sauce to the chicken filling, and set the rest aside.
Spray a medium sized casserole pan with non-stick cooking spray. Fill tortillas with chicken mixture, one at a time, rolling and placing in prepared pan until you have used up the filling. You want them to fit fairly snugly together. Pour remaining sauce over the top, and sprinkle with remaining cheese. Bake in preheated oven for 30-35 minutes, until bubbly and cheese is beginning to brown.
Notes
Filling and sauce can be made a day or two ahead. Assemble and bake the day you plan to serve them.