If you normally throw the stems from a bunch of cilantro or parsley straight into the garbage...DON'T!
2 links fresh chorizo , casings removedMeasure out ½ cup chickpeas; reserve remaining chickpeas for another use. Finely grate zest of lime into a small bowl. Cut lime in half and squeeze juice from one half into bowl with zest. Cut remaining half into wedges; set aside for serving.
Gather cilantro on a cutting board. Starting at the stem end, finely chop until you get to the leaves; reserve leaves for serving. Add stems to bowl with lime. Add yogurt to bowl and mix well. Season with salt. Heat a medium nonstick skillet over medium. Cook chorizo, breaking it up with a wooden spoon, until partially cooked, about 3 minutes. Add chickpeas and season with salt. Cook, mashing some of the chickpeas with spoon until chorizo is cooked through and crisp, 8–10 minutes. Remove from heat.
Spread yogurt mixture over tostada shells. Top with chorizo mixture, then with reserved cilantro leaves. Serve with reserved lime wedges for squeezing over.