Preheat oven to 350°F.
Lightly grease a medium round pie dish.
In a pan, heat olive oil and sauté onion until wilted.
Add celery and garlic and sauté until celery is wilted.
Add black beans, tomatoes, and cumin, and cook until heated through.
Remove from heat and add salt and pepper to taste.
In the pie pan, layer 4 corn tortilla halves with 1/3 black bean mixture (the top layer is where the cheese goes if you're using it).
Repeat layers, ending with the bean mixture.
Cover and bake for 30 minutes.
Remove from oven, and sprinkle with spring onions, cilantro, and lime juice.
