Preheat oven to 450 degrees. Heat a large skillet over medium-high. Add sausage and cook, breaking meat apart with a wooden spoon, until sausage is browned and the fat is rendered, about 5 minutes. With a slotted spoon, transfer sausage to paper towels to drain.
Add scallion whites and garlic to skillet; cook until fragrant and soft, about 20 seconds. Stir in beans and cook 1 minute. Add broth, chili powder, cumin, and salt. With a potato masher or the back of a wooden spoon, mash beans until chunky-smooth. Reduce heat to medium and cook until warmed through, 1 to 2 minutes.
Spread half the tortilla chips in a large ovenproof platter or baking dish. Top with half the beans, half the sausage, half the cheese, and half the jalapenos (if using). Repeat with remaining ingredients. Bake until cheese melts and nachos are hot, about 5 minutes. Serve nachos topped with scallion greens, salsa, and sour cream.