Stir the yogurt with salt and fresh herbs. Wash and dry the lime. Add the zest and the juice of ½ of the lime to the yogurt, stir well. Set aside. Keep the other half of the lime for later, cut it in wedges, and serve the wedges with the tostadas.
Sausages: Remove the chorizo casings. Heat a nonstick pan and add the sausages, breaking them into pieces with a wooden spoon. Fry over medium-low heat for about 3 minutes, breaking all the lumps with the wooden spoon.
Add the kidney beans and some salt.
Cook: Use a potato masher to roughly mash the chorizo and bean mixture. Cook until cooked through and rather crispy, about 7 minutes or so. Adjust the taste with more salt and some pepper, if necessary.
Prepare topping: While the sausages are cooking, shred the salad leaves, finely dice the radishes and finely chop the jalapeňos. Grate the cheese.
Top tostadas: Spread some yogurt on top of the tostadas. Divide the chorizo and bean mixture between the tostadas. Top with the shredded salad, radishes, jalapeňos and sprinkle with the cheese and chili flakes.
Serve immediately with lime wedges.