In a small bowl, whisk to thoroughly combine Spice Mix ingredients: garam masala, turmeric, cumin, cardamom, coriander, brown sugar, cayenne (optional), salt, and pepper. Set aside.
Season chopped chicken with 3 teaspoons of the prepared Spice Mix. Set seasoned chicken aside. Reserve the remaining Spice Mix.
Heat large, heavy skillet over MEDIUM heat (or MEDIUM-LOW, depending on your stovetop). Add olive oil and swirl to coat the bottom of the skillet. Add chopped onion to pan and cook, stirring frequently, until translucent; about 5-6 minutes. Add garlic, ginger, and jalapeño. Stir and cook two minutes.
Add chicken and cook, stirring almost constantly, until chicken is "almost" done. Chicken can still be pink in places, but should generally look like it's getting closer to done.
Sprinkle remaining spice mix over the top of the cooked chicken. Cook and stir 1 minute.
Stir in half-and-half, mixing to combine ingredients evenly. Reduce heat to LOW-LOW. Simmer chicken mixture uncovered for 20 minutes, stirring frequently.
Add spinach to chicken mixture.Fresh Spinach: Add spinach a handful at a time, stirring it in as it begins to wilt. Frozen Spinach: If you are using frozen spinach, do not defrost the spinach first. Just add it all at the same time, and allow it to defrost in the heat of the sauce.
Remove chicken mixture from heat. OPTIONAL: Stir in plain yogurt at this time if you are using it. Yogurt will give the curry a slightly tangy flavor. Add 2 tablespoons of butter to chicken mixture, stirring it in to melt.
Notes
*You can omit the ginger and garlic and substitute in 2 tablespoons Ginger-Garlic Paste.This recipe was developed based on memories of the Chicken Saag at Washington Square Mall, Beaverton, Oregon.