Bring a large pot of salted water to a boil. In 3 batches, add the spinach and blanch for 15 seconds. Remove spinach with a slotted spoon and shock in an ice bath. Remove and squeeze out excess water. Chop spinach and set aside.
Roast the peppers under a broiler for about 7- 10 minutes until all sides are charred black. Place peppers in a plastic bag and let rest until cool enough to handle. Peel the peppers, split in half lengthwise and discard the seeds and stems. Roughly chop and set aside.
Take turkey sausage out of casing and cook in skillet until lightly browned and crumbly. Set aside.
In a large saucepan, melt 4 tablespoons of the butter over medium-high heat. Add the onions and cook, stirring until very soft (3 to 4 minutes). Add the garlic, chili powder, black and cayenne peppers; cook, stirring for 45 seconds. Add ¼ cup of the flour and cook, stirring with a heavy wooden spoon to make a light roux (2 minutes). Gradually add the half and half, and cook, stirring until thick (2-3 minutes). Add the spinach and stir to incorporate; add the cooked sausage and mix. Remove from the heat and fold in half of the chopped peppers. Fold in the cubed queso blanco and cubed pepper jack, as well as 1 cup of shredded pepper jack; set aside. In a separate bowl, combine the shredded queso blanco, 1 cup shredded pepper jack, and grated cotija; set aside.
Microwave tortillas until soft and pliable. Place on a work surface. Place about ½ cup of the spinach mixture into the center of each tortilla and roll. Place, seam side down, in a single layer in a 9X13-inch casserole dish that has been lightly sprayed with cooking spray.
In a saucepan, melt the remaining 4 tablespoons of butter over medium heat. Add the remaining ½ cup of flour and cook, stirring constantly with a heavy wooden spoon to make a light roux. Slowly add the chicken stock, and cook, stirring until thickened. Add the cream and cook, stirring until thick. Add the remaining charred peppers and cook for 1 minute. Pour sauce over the filled enchiladas and bake for 20 minutes in a 350°F oven.
Remove from the oven and cover evenly with the remaining shredded and grated cheeses. Return to the oven and bake until the cheeses are melted and bubbly and the enchiladas are completely warmed through (10 to 15 minutes). Remove from the oven and let sit for 5 minutes before serving. Garnish with green onions, cilantro and crema fresca.