Preheat oven to 400°F. Cook turnipsin a large saucepan of boiling,salted water until tender, 10-15minutes.
Drain. Return to pan; addcream. Bring to a boil. Reduceheat and simmer until cream coatsturnips, about 4 minutes. Puréeturnip mixture in a food processoror with an immersion blender untilsmooth. Cover and keep warm.
Meanwhile, heat 1 teaspoon oil ina large ovenproof skillet overmedium-high heat. Season lambwith salt and pepper. Cook fat sidedown until brown, 5-8 minutes.
Turn lamb over. Tuck 4 thymesprigs under; place in oven. Roastuntil an instant-read thermometerinserted into center registers 125°Ffor medium-rare, 15-25 minutes.
Let rest on a plate for 10 minutes.
Drain fat from skillet; add
Marsala. Bring to a boil. Reduce heat;simmer, stirring and scraping upbrowned bits from bottom of skillet,until reduced by half, 5-8 minutes.
Meanwhile, heat remaining 2 teaspoonsoil in a large skillet over mediumheat.
Add garlic and remainingthyme sprig; stir until beginningto brown, about 1 minute; discardgarlic and thyme sprig.
Add spinachby the handful, tossing and allowingit to wilt slightly between additions.Season with salt and pepper. Cookuntil tender, about 4 minutes.
Cut lamb between bones intochops and serve with spinach,turnip purée, and Marsala sauce.