In a large bowl or gallon-size plastic bag, combine 1/3 cup plain Greek yogurt, 1 tablespoon fresh lemon juice, 1 tablespoon grated ginger (or 1 teaspoon ground ginger), 3 garlic cloves (minced), 1 teaspoon kosher salt, 1 teaspoon ground cumin, and 1/2 teaspoon paprika. Mix well. Add the diced chicken pieces and stir to coat evenly. Cover and marinate in the refrigerator for at least 1 hour, or ideally 2-3 hours for best flavor.
Heat 1 tablespoon canola oil in a large skillet over medium-high heat. Remove the chicken from the marinade (discard any excess marinade) and add to the pan. Cook for 7-8 minutes, stirring occasionally, until the chicken is cooked through. Once done, remove the chicken from the pan and keep warm by placing it in a bowl covered with foil.
In the same pan, add the remaining 1 tablespoon of canola oil. Add the chopped onion and cook for 3-4 minutes until softened. Stir in 3 garlic cloves (minced), 1 tablespoon grated fresh ginger, 1 1/2 teaspoons garam masala or curry powder, 1 1/2 teaspoons ground coriander, 1 1/2 teaspoons turmeric, 1 teaspoon paprika, 1 teaspoon ground cumin, and 1/4 teaspoon black pepper or crushed red pepper flakes. Cook for 30 seconds until fragrant. I love how the spices bloom their flavor when briefly cooked like this.
Stir in 3 tablespoons tomato paste; the mixture will be very thick. Add the can of tomato sauce or crushed tomatoes and bring the mixture to a simmer while scraping the bottom of the pan to loosen any stuck-on bits. Once simmering, reduce the heat to medium-low and cook for 3-5 minutes, until the sauce is slightly thickened.
Stir in 1/3 cup plain Greek yogurt or heavy cream until the sauce is smooth. Return the cooked chicken (with any juices from Step 2) to the pan and stir well to combine. Let everything simmer together over low heat for a couple of minutes. For an extra creamy finish, stir in 2 tablespoons unsalted butter, if desired.
Taste the sauce and season with salt and pepper as needed. Serve the chicken curry hot over cooked basmati rice, and garnish with chopped fresh cilantro or parsley. I like to add a generous handful of cilantro for a fresh, vibrant finish.