In a large pan with high sides, melt the butter over low heat. When gently simmering, carefully add the shad roe to the butter, cover, and poach gently for 10-12 minutes, turning once.
While the shad roe is poaching, make the sauce. Melt the butter in a small saucepan over low heat. Whisk in the flour and cook, constantly stirring for 2 minutes. Slowly pour in the cream, whisking as you pour to avoid any lumps and create a smooth sauce. Cook gently for 5 minutes, season with salt and pepper, then fold in the spinach to wilt.
When the shad roe is cooked, carefully slide it out of the pan and keep warm on a plate.
Spoon the sauce over the roe. Serve with steamed asparagus and lemon wedges.
