Heat oil in a skillet over medium heat. Cook onion until translucent and lightly golden. Add chicken and salsa and stir to combine. Heat until salsa is thick and chicken is warm.
Meanwhile, heat black beans and cream in a small saucepan to a simmer. Puree with an immersion blender, you can thin with a little extra warm cream if needed.
Build the tostada. Spread tostada with black bean puree, top with chicken, drizzle with sour cream, garnish with corn, tomato, lettuce and cilantro.Finish with a squeeze of lime.