1. Preheat broiler to high. Line a broiling pan or large, rimmed baking sheet with foil and mist with cooking spray. Arrange tortilla chips, overlapping slightly, in a single layer on top of foil.
2. Warm oil in a large skillet over medium-high heat. Add onion and cook, stirring frequently, until tender and translucent, about 3 minutes. Add garlic, sprinkle with chili powder and cook, stirring, until fragrant, about 1 minute longer. Stir in beans and tomatoes. Cook, stirring, until just warmed through, about 2 minutes. Stir in lemon juice and cilantro. Season with salt and pepper.
3. Spoon bean mixture onto center of chips and spread evenly. Sprinkle cheese evenly on top. Broil, watching constantly and turning pan often, until nachos are hot and cheese is bubbly, about 1 minute. Remove pan from broiler, sprinkle avocado over nachos and garnish with dollops of yogurt. Transfer to a platter and serve immediately.