In a large bowl, combine chicken, ground coriander, cumin, and salt. Toss to combine. Stir in yogurt, coating chicken evenly. Marinate for at least 10 minutes.
In a large nonstick skillet, heat 1 tablespoon olive oil over medium-high heat. Add half of chicken and cook until browned on both sides. Transfer to slow cooker. Repeat with remaining chicken.
In same skillet, add 1 more tablespoon olive oil. Add onions, garlic, and red pepper flakes and saute until onions are softened and translucent, about 5 to 7 minutes.
Add 1 teaspoon salt ,garam masala, and ginger and cook 1 more minute. Add tomatoes and sugar, turn heat to high, and bring to a boil. Pour over chicken in slow cooker. Cover and cook for 5-6 hours on LOW.
Whisk together cornstarch and 1/4 cup heavy cream and add to crockpot along with remaining heavy cream and peas. Stir to combine. Cover and continue to cook 10 minutes on HIGH.