In a large wok, heat the oil over medium high heat. Saute the garlic and ginger for one minute until fragrant and beginning to brown. Add the mushrooms, and toss to coat. Add the soy sauce and stir to combine. Let cook for 5 minutes or so, until softened. Drizzle with sesame oil and toss to coat. Season with a little salt if desired. Remove the mushrooms to a bowl. (These can be made in advance.)
Heat the remaining oil in the same wok. Add the spinach, and stir fry until wilted, 2 – 3 minutes. Add in the rice, mushrooms, and chicken (if desired), stirring to combine. Season to taste with soy sauce.