Preheat the oven to 400°F (200°C).
In a large baking dish, combine the chicken, Greek yogurt, chopped garlic, grated ginger, garam masala, 1/2 teaspoon turmeric, cayenne pepper, and 1 teaspoon salt; toss to coat and let sit 5 minutes.
Add the chopped onion to the chicken, pour in the coconut milk, and stir in the tomato paste until the mixture is creamy and evenly combined.
Arrange slices of the cold salted butter over the top of the chicken, sprinkle with sesame seeds, and dot with the additional 2 tablespoons salted butter if using; season with black pepper.
Bake uncovered for 30–40 minutes, until the chicken is cooked through and the sauce is bubbling; remove from oven and sprinkle with chopped cilantro or mint.
While the chicken bakes, make the cucumber-ginger salad: toss the sliced cucumbers with the pickled ginger (and 1–2 tablespoons of the ginger juice if desired), chopped cilantro or mint, sliced jalapeño, lemon juice, and salt to taste.
For the golden butter, melt remaining butter and chili flakes in a skillet over medium heat and cook until the butter begins to brown, then remove from heat.
To serve, spoon the baked chicken and sauce over steamed rice, add a side of naan, top with the cucumber salad and drizzle with the golden browned butter.
