1. In blender or food processor, puree yogurt, cottage cheese, garlic, chili powder, sugar and cornstarch. Set aside.

2. Melt butter in a 2 quart pan over medium high heat. Add flour and 1/3 cup water, stir until just bubbly. Whisk in broth. Bring to a boil, stirring. Remove from heat. Let cool 5 minutes. Whisk yogurt mixture into flour mixture and then stir in ch iles. Cover the bottom of a 9x13 inch baking dish with 1/3rd of the mixture.

3. Dip tortillas one at a time into hot water. Drain briefly. Cut into 1 inch wide strips. Scatter half the tortilla stips over the yogurt-flour mixture in the baking dish. Cover with all of the chicken and onion, and half of the jack cheese. Spread half of the remaining yogurt-flour mixture over this. Top with remaining tortilla stips, followed by the remaining yogurt-flour mixture and jack cheese.

4. Cover dish tightly with foil and bake in a 400 degree oven for 30 minutes. Uncover. Continue to bake until mixture is golden brown on top and appears firm in the center when the dish is gently shaken (15 to 20 more minutes).