Combine the 1/2 cup picante sauce and the about 30 oz of red enchilada sauce in a small saucepan and simmer over low heat, stirring occasionally, until reduced to about 2 1/2 cups, about 25–30 minutes.
While the sauce reduces, shred the cooked chicken, measure the sour cream, and grate the cheese.
When the sauce is reduced, transfer the shredded chicken to a mixing bowl and gently stir in 1 cup of the sauce, 1 cup sour cream, and 1 1/2 cups of the shredded Mexican blend cheese until evenly combined.
Lay the 10 tortillas flat and divide the chicken filling evenly among them, placing the filling in the center of each tortilla and rolling them up tightly.
Spray the slow cooker insert or the bottom of a 10x12-inch casserole dish with nonstick spray and spread about 1/4 cup of the reserved sauce over the bottom.
Arrange the rolled enchiladas seam-side down in the slow cooker or casserole dish, pressing them snugly together to fit all 10.
Pour the remaining sauce evenly over the enchiladas and sprinkle the remaining 1 1/2 cups shredded cheese on top.
For slow cooker: cook on High for 2 1/2–3 hours until bubbling and cheese is melted; if excess moisture accumulates under the lid, carefully remove the lid partway through and discard the collected liquid.
Optional oven finish for slow cooker: if desired, transfer the insert (if oven-safe) or place enchiladas on a baking sheet and brown the top in a 400°F (200°C) oven for 10–15 minutes; do not broil if using a non-broil-safe insert.
To bake entirely in the oven: preheat to 375°F (190°C) and bake the assembled enchiladas in a 10x12-inch dish for 30–40 minutes until hot and slightly browned.
Serve hot garnished with thinly sliced green onions and additional sour cream at the table if desired.