Pickle the jalapeño: Whisk vinegar, sugar, and salt until dissolved. Add sliced jalapeño and cilantro, toss, and set aside.

Soften the onion: Mix red onion with the zest and juice of one lime and a pinch of salt. Massage gently until slightly softened.

Prepare the cucumber: Crack the cucumber with the flat side of a knife, slice into one-inch pieces, salt, and rest for ten minutes. Squeeze out excess liquid.

Make the dressing: Warm olive oil over medium-low heat. Add garlic and cook for two to three minutes until golden. Blend with jalapeño, remaining lime, agave, miso, cilantro, and salt.

Combine: Add beans, cucumber, and remaining cilantro to the onion. Pour over the dressing and toss gently.

Serve: Mash avocado with salt and spread over tostadas. Top with bean salad and pickled jalapeño.