Preheat the oven to 375 degrees F. Grease a 9 by 9 by 2-inch square baking dish. In a large skillet, heat the oil. When the oil is hot, add the onions and saute for 3 to 4 minutes. Add the ground beef and continue to cook for 3 to 4 minutes, stirring constantly. Add the diced meat and continue to cook for 3 minutes. Stir in the garlic, chili powder, and cumin. Season with salt and pepper. Stir in the tomato paste. Stir in the tomatoes and stock. Simmer the mixture for 30 minutes, stirring occasionally. Remove from the heat and cool. Spoon a couple of tablespoons of the chili mixture on the bottom of the prepared pan. Lay two tortillas on top of the chili mixture. Sprinkle the top of the tortillas with the Cheddar and Jack cheese. Repeat the layering process until all of the chili, tortillas and cheese are used. (The final layer should be cheese) Place the pan in the oven and bake until the cheese has melted, about 10 to 12 minutes. Remove from the oven and cool slightly before slicing.