Heat 1 T oil in heavy saucepan over medium-high heat. Add shallots; cover and cook until soft and brown, about 3 minutes. Add stock, wine, and brandy. Boil until slightly reduced, 6 minutes. Add cream; boil until reduced to 3/4 cup, about 6 minutes. Set sauce aside.
Melt butter in heavy large skillet over medium-high heat. Add garlic and saute 1 minute. Add 1 bag of spinach and toss until slightly wilted, about 1 minute. Add remaining spinach and toss just until wilted, about 2 minutes. Season to taste with salt and pepper. Set aside.
Heat remaining 1/2 T oil in another heavy large skillet over high heat. Sprinkle steaks with salt and pepper. Saute to desired doneness, about 4 minutes per side for rare.
Divide spinach among 4 plates. Place 1 steak on each mound of spinach. Add sauce, mustard, and tarragon to skillet steaks cooked in; boil until slightly thickened, scraping up browned bits from bottom of pan, about 1 minute. Season sauce with salt and pepper. Spoon sauce over steaks and serve.