Toss ingredients for corn and radish salad in a medium mixing bowl. Stir well to combine. Taste and season additionally if needed.
To prepare the beans, heat olive oil in a large saucepan. Add the onions and garlic with a sprinkle of salt. Cook, stirring occasionally, until the onions have softened and the garlic is fragrant, about 6-8 minutes. Add the cumin and cook for another 30 seconds. Pour in beans, salsa and vegetable stock. Stir then cover and reduce heat to a simmer. Cook for 10 minutes, then remove lid and use the back of a fork or masher to mash half the beans. Remove from heat, season generously with salt and pepper and cover until ready to serve.
Combine cilantro and lime juice in a food processor and pulse until fine. Add sour cream and salt and puree until smooth. Taste and adjust seasoning as needed.
To warm the tortillas, heat a cast iron skillet over medium heat and warm each tortilla, flipping occasionally. Fold in a towel to keep them warm.
Spread the beans down the middle of each tortilla, then top with corn salad. Top with sliced avocado, salsa and a side of crèma.