Put all (or some--you may need to do this in batches, depends on the size and power of your food processor) the duxelles ingredients into a food processor. Process until small pieces result. The goal is to achieve a finely chopped texture, not puree the mushrooms.
Lightly grease a skillet with coconut oil cooking spray. Add the duxelles and sauté until lightly browned and most of the water has evaporated. It should be crumbly and somewhat dry.
Lay a sheet of wide plastic wrap (or two sheets of regular plastic wrap, side by side, overlapping by several inches) on the counter.
Place 4 or 5 slices of prosciutto, long sides overlapped by ½-inch or so, on the plastic wrap.
Spread half of the duxelles over the center of the prosciutto
Preheat a skillet over medium-high heat. Melt the butter then brown just the outside of the meat pieces on all sides, including the top and bottom. Transfer to a plate.
While the meat is still hot, brush it all over with the dijon mustard.
Place one piece of meat atop the duxelles, towards one end of the rectangle.
Roll the whole thing up using the plastic wrap to help you along, but being careful not to roll the plastic wrap into the wellington.
Twist the ends of the plastic wrap tightly so that the wellington looks shaped like a candy roll.
Pop in the fridge. Repeat process with the remaining prosciutto, duxelles, meat, and mustard.
Heat oven to 400° Fahrenheit. Line a baking sheet with parchment paper or a silicone baking mat; set aside.
Lightly spray a small saucepan with nonstick coconut oil cooking spray. Heat on medium-low heat. Add the cheese and cream cheese, stirring constantly, until melted, completely blended, and stretchy. Alternatively, you can add the cheese and cream cheese to a glass bowl and microwave it, 30 seconds at a time, stirring in between, until melted, well blended, and stretchy.
Meanwhile, in a medium bowl, mix together the Baking Blend, parsley, garlic powder, onion powder, and salt.
Add the powdered mixture to the cheese mixture. Begin stirring it. Add the egg and stir the rest of the way. If you like, once the mixture has cooled enough you can touch it (but still very warm), you can knead it to mix it the rest of the way. A stretchy, smooth, still warm dough should result.
Lay another wide sheet of plastic wrap onto the counter (or two regular width sheets, overlapped by several inches). To keep it from moving while rolling the dough, you can put a tiny bit of water on the counter first so that it "sticks" to the counter.
Roll half the pastry dough out onto the plastic wrap until a ¼-inch thick rectangle about 14 inches long by 12 inches wide (give or take--depends on size of meat) results.
Remove one of the wellingtons from the fridge and carefully remove and discard of its plastic wrap.
Place the wellington roll towards one of the short ends of the pastry dough and roll up tightly, using the plastic wrap to help (but being careful not to get the plastic wrap rolled into the wellington). Cut off any excess dough as necessary to keep the dough from overlapping much.
Twist the ends of the plastic wrap so that the wellington, with pastry and all, is pressed into a candy roll shape.
Pop in the fridge for about 10 minutes.
Repeat steps 7-10 with the remaining dough and wellington.
Once both wellingtons have done their final chill, remove and discard the plastic wrap and place the wellingtons on the prepared baking sheet.
Using the blunt side of a knife, make line indentations of your choosing on the dough (this step is optional but makes it look fancy).
Brush the pastry with egg yolk. Sprinkle on some coarse mineral salt.
Pop in the oven and bake for 25 to 45 minutes, give or take, until crust is golden brown and the meat has reached your desired temperature.
Allow to cool for 10 minutes before slicing and serving.