Heat a nonstick skillet over medium heat. Add butter; melt. Season tuna steaks with salt and pepper; place in the skillet and cook 3 to 4 minutes per side. Transfer to a serving plate; keep warm.
Add half-and-half and cheese to the skillet; stir until incorporated. Stir in lemon juice. Reserve 2 tablespoons sauce; set aside. Add spinach to the skillet; cook and toss with sauce until wilted, 1 to 2 minutes.
Divide spinach between two plates; top with tuna steaks. Serve reserved sauce over tuna steaks.
