This is the author's version of spanakorizo, a popular Lenten dish of rice and greens flavored with fresh dill. Reserving half of the spinach and mustard greens to add at the end keeps the color fresh and vibrant. Though you start with a lot of greens, they cook down quite a bit.
Heat oil in a large Dutch oven over medium heat. Add onion; cook 5 minutes or until tender, stirring occasionally. Add garlic; cook 1 minute, stirring constantly. Add rice; cook 2 minutes, stirring frequently. Add water, 1 1/2 tablespoons dill, half of spinach, and half of mustard greens. Bring to a boil; cover, reduce heat, and simmer 20 minutes or until liquid is absorbed. Stir in remaining spinach, remaining mustard greens, 1 1/2 tablespoons dill, lemon juice, salt, and peppers; cook 30 seconds, stirring constantly.