Preheat oven to 400°F. Line rimmed baking sheet with parchment paper, or spray with cooking spray. Spray mushroom caps with cooking spray, and sprinkle gill sides with salt and pepper, if desired. Arrange mushrooms gill-sides down on prepared baking sheet. Roast 10 minutes, or until mushrooms begin to soften. Turn mushrooms over.Meanwhile, heat 2 tsp. oil in large nonstick skillet over medium heat. Add leeks, garlic, and thyme, and sauté 4 minutes, or until leeks are tender. Add 2 large handfuls spinach at a time, and cook 8 to 10 minutes, or until spinach is wilted and most liquid has evaporated, stirring often.Stir in goat cheese and pine nuts until cheese is melted. Remove from heat, and season with salt and pepper, if desired. Fill mushrooms with spinach mixture, mounding on top and packing filling down with hands. Bake 25 minutes, or until filling browns on top and mushrooms are fork-tender.Cook farro in large saucepan of boiling salted water 20 minutes, or until just tender, stirring occasionally. Drain. Transfer to large bowl. Stir in lemon zest, lemon juice, and remaining 2 tsp. olive oil. Season with salt and pepper, if desired. To serve, spoon 1/2 cup farro mixture onto each plate; top with 1 stuffed mushroom.