Preheat oven to 200 degrees F. Heat oil in skillet set over medium-high heat. Season scallops with salt and pepper. Add to skillet and cook 2 minutes on each side. Remove from skillet, place on a heatproof plate, drizzle with lemon juice and then keep warm in the oven. And mushrooms to the skillet and cook until tender. Add garlic, stock and tarragon and bring to a simmer.
Add spinach and cook until it just wilts. Divide mixture among 4 plates. Divide and arrange scallops on top of spinach mixture. Garnish with lemon wedges and tarragon sprigs. Serve with rice or pasta.
Note: Fresh tarragon, a liquorice-tasting herb, is available in the produce department.