Cheesy Puffy Tacos

Preheat oven to 350º. Line two baking sheets with parchment paper.  In a large skillet over medium-high heat, heat oil. Add ground beef and onion. Cook, breaking up beef with a wooden spoon, until golden and cooked through and onion is softened, about 8 minutes. Drain excess fat.  Add diced tomatoes and taco seasoning. Bring to a simmer, and cook until liquid is evaporated, about 2 minutes. Remove from heat.  Unroll one tube of crescent rolls on a work surface and pinch diagonal seams together. Cut into 8 squares and top each square with about ¼ cup taco beef mixture and some cheddar and Monterey jack. Fold two opposite corners of each square over the filling and press to seal.  Arrange on prepared baking sheet. Repeat with the remaining crescent rolls, taco mixture, and cheeses. Bake until golden and cooked through, rotating trays halfway through, about 12 minutes. Serve warm or at room temperature.