Toss zucchini and 1/2 tsp salt in large bowl. Let stand 5 minutes. Transfer to a sieve. Press out excess liquid; place zucchini in a dry bowl. Mix in egg yolk, 1/2 cup flour, feta, and 1/2 tsp salt. Mix in parsley, onions, and dill. If batter is wet add more flour by spoonfuls. Heat 2 tbsp olive oil and 2 tbsp corn oil (or high heat oil) in a large skillet over medium heat. Working in batches drop batter by rounded tablespoonfuls into skillet. Fry patties until golden brown, 5 minutes per side, adding more oils as needed. Transfer to paper towels. Serve with yogurt.