In a bowl, combine salsa and sour cream.
Stir together until well-incorporated. Cover the bowl and chill in the fridge until ready to assemble.
Heat up oil in a skillet. Add the leftover brisket to warm and soften. Stir in cream cheese until it melts and everything is well-combined.
Preheat the oven to 355degrees. Reserve a cup and a half of the salsa and sour cream mixture.
Spread half of the reserved salsa mixture in the bottom of a casserole dish. Place some of the brisket meat , salsa mixture, and shredded cheese on flour tortilla. Fold the bottom then roll tightly.
Place enchiladas into the pan. Repeat process for the remaining tortillas.
Spread the remaining salsa mixture on top.
Sprinkle more shredded Mexican cheese on top. Bake it in the oven for 30 minutes.
Let the enchiladas cool down for 5 minutes before serving.