Add lentils and broth to a medium saucepan; bring to a boil. Reduce heat and simmer, partially covered, until lentils are soft, 30–40 minutes. Stir in 1 Tbsp. lemon juice and capers. Season to taste with salt and pepper.
Meanwhile, preheat oven to 425°F. Remove toughest outer leaves from cauliflower (leave on any tender inner leaves). Trim stem to create a flat base. Holding cauliflower upright on stem, slice into 4 equal slabs to create steaks. Place steaks and florets on a rimmed baking sheet; Brush with 2 Tbsp. of the oil, 1/2 tsp. salt, and 1/4 tsp. black pepper. Bake for 25 to 30 minutes, carefully flipping/tossing once halfway through, under tender. Brush with 2 Tbsp. harissa; bake for 5 more minutes.
Combine beans, remaining 2 Tbsp. harissa, garlic, remaining 2 Tbsp. lemon juice, 3 Tbsp. oil, 1/2 tsp. salt, and 1/4 tsp. black pepper in a food processor; blend until smooth.
Divide harissa "hummus" evenly among each of 4 plates. Top evenly with lentils and one cauliflower steak. Brush additional harissa over cauliflower, if desired. Garnish with fresh mint, pine nuts, and an extra drizzle of olive oil.