Preheat oven to 350°F (180°C).
In a medium bowl combine chicken, 1 cup shredded cheese, softened cream cheese, onion powder and cumin. Mix until well combined.
Spoon about 1/4 cup (60mL) chicken mixture into center of each flour tortilla and roll up tightly. If using corn tortillas read about warming them first in notes below. Place seam side down, in a single layer in a greased 13- by 9-inch (33 by 23 cm) baking pan (use metal or foil pan if freezing for later).
Time to make the white sauce now. In a medium saucepan over medium heat, melt butter. Sprinkle cornstarch overtop and cook, whisking constantly, for 2 to 3 minutes, until foamy and simmering (do not let brown).
Add chicken broth; cook, whisking constantly for 3 to 4 minutes until sauce thickens and starts to simmer. Remove from heat. Allow to cool slightly for 5 minutes.
Add sour cream and salsa verde to white sauce, whisking until well combined. Stop right now and give the sauce a taste. Be the chef! If you want it spicier, add more salsa verde. If you want more flavor add a dash of onion powder or celery salt. When it's jussst right you pour the sauce over your enchiladas. Spread to cover them all.
Top with remaining cheese.
Bake uncovered in oven for 20-25 minutes, until sauce is bubbly and cheese on top is melted.
Then turn your broiler on high and broil for 1-2 minutes until cheese on top is bubbly and starting to get golden in spots. Be careful not to burn it! Then remove from heat, let cool 5 minutes and serve.