In a large bowl, combine 1/4 cup Greek yogurt, lemon juice, 1 teaspoon garam masala, 1/2 teaspoon ground cumin, 1/2 teaspoon ground coriander, 1/4 teaspoon turmeric powder, 1/4 teaspoon chili powder, and 1/2 teaspoon kosher salt.
Add the chicken cubes to the marinade, stir to coat evenly, cover, and refrigerate for at least 1 hour or up to 4 hours.
Preheat the grill or grill pan over medium-high heat and lightly oil the grates.
Thread the marinated chicken pieces onto skewers and grill for 5-7 minutes per side until cooked through and slightly charred. Remove from heat and set aside.
In a large skillet, heat 1 tablespoon vegetable oil over medium heat.
Add the chopped onion and cook, stirring occasionally, until soft and golden, about 7 minutes.
Add the minced garlic and grated ginger and cook for 1 minute until fragrant.
Stir in the remaining 1 teaspoon garam masala, 1/2 teaspoon ground cumin, 1/2 teaspoon ground coriander, 1/4 teaspoon turmeric powder, 1/4 teaspoon chili powder, and 1/2 teaspoon kosher salt. Cook for 1 minute to toast the spices.
Pour in the crushed tomatoes and simmer for 10 minutes, stirring occasionally.
Reduce the heat to low and stir in the remaining 1/4 cup Greek yogurt and 1/2 cup low-fat milk until the sauce is smooth and creamy.
Add the grilled chicken pieces to the sauce and simmer for 5 minutes to combine flavors and heat through.
Remove from heat and sprinkle with chopped fresh cilantro before serving.