To make sauce: Bring all ingredients to a simmer in a saucepan over med heat. Whisk to combine, then remove from heat. Season with salt and pepper if desired.
To make filling: Heat olive oil in a separate saucepan over med heat. Add onion and saute 5 min or until soft. Add sweet potatoes, tomatoes, salsa,, garlic, chili, and 1/2 cup water. Bring to a boil. Reduce heat to medium-low and simmer 30-140 minutes or until sweet potatoes are soft. Mash mixture with potato masher until combined. Add black beans and cook 5 minutes. Stir in half the queso fresco and remove from heat.
To assemble: Preheat oven to 350 degrees. Brush a 13x9 baking dish with oil. Spread 1/2 cup sauce in bottom. Fill tortillas with filling. Roll, and pack close together, seam side down. Top with remaining sauce and queso fresco. Bake 15 minutes.
Adjust the oven to broil for 5 minutes or until cheese is browned and bubbly. Let rest a few minutes before serving. Garnish with avocado and sour cream.