Add 1/4 cup oysters to a small saute pan set over high heat and cook until oysters start to curl, about 3 minutes.
Add cold butter and cook, stirring constantly, until melted.
Transfer to a pre-heated coffee mug, and puree with an immersion blender until creamy.
Reserve in a very warm place or a double boiler at just under simmering.

Season fish filets with salt and pepper and dust with flour.
Preheat a large - 12-inch or larger- saute pan, or 2 smaller saute pans over medium-high heat.
Add 1 tablespoon olive oil and 1 tablespoon butter.
Add fish and cook until flakey, about 4 minutes per side. 

Meanwhile, set another pan over medium-high heat and saute shiitakes in remaining olve oil for about 3 minutes, stirring often.
Add tasso and spinach and saute until spinach wilts, about 2 minutes, stirring often.
Add remaining oysters and cook just until edges curl, about 3 minutes.
Remove from heat and keep warm until ready to serve.

Presentation
Divide oyster/spinach/tasso mixture amongst 4 large plates.
Top each plate with 1 ounce of pureed oyster butter.
Place 1 piece fish atop each plate.
Serve.