Preheat your oven to 400°F (200°C). In a skillet over medium-high heat, cook ground beef and diced onion until browned, about 8 minutes. Drain excess fat. Add taco seasoning and 3/4 cup water, simmering until liquid reduces, about 5 minutes.

Line two large baking sheets with parchment paper. Spread tortilla chips evenly across both sheets. Spoon dollops of refried beans, then layer the seasoned beef, and sprinkle generously with shredded cheese.

Bake in the preheated oven for 15 minutes, or until the cheese is melted and bubbly. Watch closely to avoid burning.

Sprinkle diced Roma tomatoes and sliced green onions over the baked nachos. Serve hot directly from the baking sheets or transfer to a platter.