Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the chicken cubes and sear until browned on all sides, then remove chicken from the pan and set aside.
In the same skillet, add the diced onion and sauté for 4-5 minutes until translucent.
Add the minced garlic, grated ginger, and red curry paste; cook for 1-2 minutes until aromatic.
Whisk in the peanut butter, coconut milk, soy sauce, and brown sugar until the sauce is smooth and well-combined.
Return the chicken to the skillet and simmer over medium-low heat for 10-12 minutes or until the chicken is cooked through and the sauce has thickened.
Stir in the lime juice and baby spinach, cooking for an additional 1-2 minutes until the spinach has wilted.
Serve hot over steamed jasmine rice, garnished with fresh cilantro and crushed peanuts.