Add the sliced chicken to a medium bowl and toss it together with 3 Tablespoons water, 2 teaspoons reduced sodium soy sauce, and 2 teaspoons cornstarch. Let sit for 10 minutes.
Add 1 Tablespoon of the oil to a saute pan or large skillet over medium-high heat. Add the chicken (leave excess liquid behind in the bowl) and cook until lightly browned, and no more pink remains. Remove the chicken and any juices from the pan and set it aside in a clean bowl.
Wipe the pan clean with a paper towel and return it to medium-high heat. Add the remaining 1 Tablespoon of canola oil. Add 1 Tablespoon grated fresh ginger, minced garlic, and 2 Tablespoons red curry paste to the pan and cook until fragrant, and just barely beginning to brown, about 30-60 seconds. Stir in 1 can light coconut milk, 1/4 cup brown sugar, 2 Tablespoons peanut butter, and 2 Tablespoons fish sauce.
Bring to a boil, then reduce to a simmer. Cook for about 10 minutes, until slightly thickened, stirring often. Stir in the cooked chicken and any accumulated juices, and one-half to three-quarters of the chopped peanuts and cook for another minute or two. Stir in 8 ounces baby spinach (you may need to do this in batches depending how big your pan is) and cook until wilted, about 2 minutes.