In a large Mixing Bowl, combine the ½ cup full-fat plain Greek yogurt, 1 tsp 1 tsp garam masala, ½ tsp ½ tsp ground cumin, ½ tsp ½ tsp turmeric, and ½ tsp ½ tsp kosher salt. Add the 1 ½ lb boneless skinless chicken thighs cut into 1½-inch pieces and toss to coat thoroughly. Cover and marinate for at least 20 minutes at room temperature, or refrigerate for up to 24 hours for deeper flavor.

Melt 1½ tbsp 3 tbsp unsalted butter divided in a large Skillet over medium-high heat. Working in batches if needed, add the marinated 1 ½ lb boneless skinless chicken thighs cut into 1½-inch pieces in a single layer. Sear for 3–4 minutes per side until lightly charred and just cooked through. Remove the 1 ½ lb boneless skinless chicken thighs cut into 1½-inch pieces to a plate and set aside — it will finish cooking in the sauce.

Rinse peppers under cold water and pat dry. Slice off tops and bottoms, stand upright, and cut down along the inside to remove seeds and white ribs. Slice lengthwise into ½-inch strips; halve crosswise for shorter pieces. Keep sizes uniform so everything cooks evenly. If adding onion, slice it now. Measure out olive oil, salt, and pepper. Have everything ready before the pan goes on — sautéing moves fast.

Heat a large Skillet over medium heat. Add 1 tbsp olive oil per 2 peppers; warm 30–60 seconds until shimmering. Add pepper strips in a single layer — work in batches if crowded. Season with salt and pepper, stir to coat. Cook over medium heat, stirring every 2–3 minutes, for 6–10 minutes. At 6 minutes: slightly softened with snap. At 10 minutes: tender throughout with light color. For very soft peppers, cook 30–40 minutes over medium-low, stirring occasionally. Add garlic halfway through cooking. If browning too fast, add a splash of white wine or water and briefly cover to steam.

Carefully transfer the sauce to a Blender (or use an Immersion Blender directly in the Skillet) and blend until smooth. Return the sauce to the Skillet over medium-low heat. Stir in the 1 cup heavy cream and bring to a gentle simmer. Add the seared 1 ½ lb boneless skinless chicken thighs cut into 1½-inch pieces back to the pan and simmer for 8–10 minutes, until the 1 ½ lb boneless skinless chicken thighs cut into 1½-inch pieces is fully cooked through and the sauce is velvety and coats the back of a spoon.