In a bowl, combine the black beans, corn, onion, tomatoes and half the coriander. Pour over the lime juice. Season and set aside.
Make the dip: mash the avocado in a bowl, mix in half the yogurt and the rest of the coriander, then season and set aside.
Brush a frying pan with oil and place over a medium heat. Press one tortilla into the pan, top with the Cheddar and half the bean mixture. Spread the remaining tortilla with the other half of the yogurt and press, yogurt-side down, on top.
Cook for 2-3 minutes on each side (invert on to a plate to turn it over), until golden and the filling is hot. Cut into wedges and serve with the dip.