Heat oil in a skillet on medium heat. Saute`mushrooms and onions until mushrooms soften and onions are translucent. Add garlic and seasonings to taste; stir. Cook an additional 2 minutes. Turn off heat and set vegetable mixture aside.
In a large pot, bring the water (or vegetable broth) to a boil. Stir in the rice and stir. Cover pot and reduce heat to low. Cook, stirring from time-to-time, for 30 minutes. (FYI: Brown rice generally takes about 45 minutes to cook.) As liquid begins to evaporate, add a little more water or vegetable broth.
Stir in the vegetable mixture, lemon juice and spinach. Cook, covered, another 10-15 minutes, or until the rice is completely cooked. Taste and adjust seasonings, if necessary.
Serve with lemon wedges, if desired.