Heat the oil in a skillet over medium heat. Add the onion and garlic and cook until beginning to soften, 2 to 3 minutes. Stir in the cumin and cook, stirring, until fragrant, about 1 minute. dd the tomato and black beans and cook, stirring occasionally, until the mixture has thickened slightly, 5 to 7 minutes. Season to taste with salt and pepper, then set aside to cool. Once cooled, stir the avocado, cilantro, and lime juice into the black bean mixture.
Spoon mixture into each chip, and top with sour cream and cheese if desired. Serve immediately to prevent the chips from getting soggy.
Filling can be prepared up to 4 hours ahead if refrigerated. Let come to room temperature before assembling and serving.