Blend 1 1/2 cup black beans in a food processor until beans are almost a paste. Add green bell peppers, onions, and garlic and process until vegetables are finely chopped.
Transfer black bean mixture in a large bowl. Add remaining black beans, flax seed mixture, salt, spices, harissa, and bread crumbs. Mix until ingredients are evenly distributed. Cover and refrigorate for 30 minutes.
For sliders, form into 8 equal patties. For burgers, form until 4 equal patties. Freeze for at least 1 hours (or until needed).
Heat grill on high, ensure grill is well oiled. Grill patties until they have desired grill marks and are heated through.
Toast buns. Spread hummus on base of bun, top with lettuce leaves and grilled patties on top of that. Finish with avocado, onions, and tomato slices. Top with bun crown. For appetizer serve in plates of four sliders, full sized burgers can stand alone.
Can make burgers ahead of time and freeze until needed.
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