Preheat oven to 230ºF.

Open leg of lamb and cut into 4 large pieces. Season all over with salt, pepper and coriander. Heat a large pot over medium high heat, and sear the lamb until golden brown on all sides.

Deglaze with pan with the white wine and reduce liquid by three quarters. Add the citrus zest and juice and add to the pan, along with garlic cloves, ancho chiles, and tomato paste. Add enough water to cover lamb by one inch. Cover pot with foil and place in pre-heated oven. Bake until fork tender, about 3 hours.

Remove lamb from liquid; reserve liquid for other use. Shred lamb and reserve, keeping warm. Moisten with reserved cooking liquid if it seems too dry.

Meanwhile, cover the bottom of a small frying pan with oil; heat over medium high heat. Fry the corn tortillas, one at a time, turning once, until crisp and golden on both sides. Drain on paper towels and hold.

Sauté black beans over medium heat with shallot and 1 tablespoon chopped garlic for 2 minutes. Mash beans with back of a fork while hot.

In small mixing bowl add remaining tablespoon garlic, onion, tomato, jalapeño, cilantro. Season to taste with salt and pepper. Dice the avocado into small pieces and add to tomato/onion mixture.

To assemble, divide the black bean mixture between the two tortillas, spreading evenly. Add the shredded lamb and lettuce and top the tostada with the avocado/tomato mixture.