In a blender or food processor, pulse garlic until minced. Add remaining ingredients and pulse until harissa is well combined. Taste and adjust seasoning as desired.
Preheat grill pan or regular pan over medium-low heat. Brush outsides of bread with olive oil. Place on piece of bread, olive oil side down, in the grill pan and smear on 2 tablespoons of harissa. Layer with sliced avocado and cheese. Smear 1-2 more tablespoon of harissa on remaining piece of bread and place olive oil side up.
Cook on each side until crisp and cheese has melted.
Notes
*To roast a bell pepper, stick under a broiler, rotating occasionally until pepper is soft and charred.