Heat a drizzle of oil in a medium pot over medium-high heat. Add onion and bell pepper; cook until softened and lightly browned, 3-5 minutes.
Stir in tomato paste, curry powder, paprika, half the garam masala, and remaining garlic until fragrant, 1 minute. TIP: Love the earthy warmth of garam masala? Add more if you like!
Stir in chickpeas, coconut milk, stock concentrate, ¼ cup water, and ½ tsp sugar. (For 4 servings, use 1⁄3 cup water and 1 tsp sugar.) Bring to a simmer, then reduce heat to low; cook until thickened, stirring occasionally, 4-5 minutes. TIP: If curry seems too thick, stir in a splash of water.
Remove from heat; stir in 1 TBSP butter (2 TBSP for 4) until melted. Season generously with salt and pepper.
Rinse shrimp under cold water. Pat shrimp or chicken dry with paper towels; season with salt and pepper. Add to pan along with onion. Cook, stirring occasionally, until cooked through, 4-6 minutes. Cook through the remainder of this step as instructed.