Put the shrimp in a large pan with enough water to cover; bring to a boil.
Remove from heat and let stand for 1 minute.
Drain into a collander, peel, and devein; set aside.
Cook the spinach by following the package directions; drain in a collander, squeeze almost dry and set aside.
In a medium saucepan, melt 6 tablespoons butter over moderate heat.
Add in the flour; whisk constantly for about 2 minutes or until very smooth.
Gradually add in the half-and-half, whisking for 5 minutes or until the sauce thickens.
Season to taste with salt and pepper, whisk and remove from heat.
Butter six to eight 4-oz. ramekins; set aside.
In a skillet, melt the remaining 2 tablespoons butter over medium heat.
Add in the scallions; stir/saute 1 minute.
Add in the sherry, cook until almost evaporated; stir onion mixture into cream sauce.
Add 1/2 cup cream sauce to the spinach mixture (adding more sauce, if necessary, for a creamy but not soupy mixture).
Divide half the spinach among the prepared ramekins; layer equal amounts of shrimp over the spinach; sprinkle each with some of the lemon juice; cover each with equal amounts of the remaining spinach mixture.
Sprinkle cheese over the top of each and bake on a heavy baking sheet in a 350° oven for about 10 minutes or until golden.