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.
In a large skillet, heat olive oil over medium-low. Add onion and sauté until translucent, 6-7 minutes. Add cauliflower and continue to cook for another 5-6 minutes. Stir in lentils, stewed tomatoes, 3/4 cup of the harissa, and 1 cup vegetable broth. Bring to a boil, reduce to a simmer, cover, and let cook until lentils are tender, 20-24 minutes.
Serve over grains.
Notes
*To roast a bell pepper, stick under a broiler, rotating occasionally until pepper is soft and charred.