In a saucepan combine the prosecco, white wine vinegar, shallots, peppercorns, saffron, and vanilla bean. Bring to a boil over medium high heat then turn down to a simmer. Allow to simmer until the liquid has almost completely reduced. You should be left with a couple tablespoons of syrupy liquid.
Add the heavy cream and whisk together. Turn the heat down to very low and begin to whisk in the butter. Remove the vanilla bean if it gets in the way while whisking. Add the butter a piece at a time whisking until it's fully incorporated before adding the next piece.
When all of the butter has been incorporated add 1 tsp kosher salt. Strain the sauce into a clean saucepan and cover. The sauce can be kept warm over a very low heat or gently warmed up later. Do not let the sauce boil or it may split.
Place a large skillet over medium high heat. Add 1 tbsp of olive oil and the minced garlic and sauté briefly. Begin to add the baby spinach in handfuls adding more as it begins to wilt. Toss the spinach with tongs to allow it to wilt quickly. Once all of the spinach has been added season with salt and pepper. Cover and keep warm.
Ensure the scallops are thawed and as dry as possible. Place the thawed scallops on a paper towel-lined plate. Pat dry with more paper towels.
Place a stainless steel pan over high heat.*see note Allow the pan to heat up. Do not add the scallops to a cold pan or they will stick. Check that the pan is sufficiently hot by sprinkling a few drops of water into the pan. They should dance around the pan and look a little like mercury balls.
Add the remaining 2 tbsp olive oil and quickly tilt the pan cover the whole pan with the oil. Add the scallops in a single layer ensuring they aren't touching. Sear the scallops until the bottom is golden brown and they release from the pan, no more than 2 to 3 minutes. Flip the scallops over and cook just until they are cooked through - another minute or two.
Remove the scallops to a plate and cover to keep warm.
Warm the beurre blanc and spinach if they have cooled down. Spoon a bed of spinach into a scallop shell or a small gratin dish. Arrange the scallops on top of the spinach and drizzle generously with the saffron vanilla beurre blanc. Enjoy!
Notes
*You will get a better sear if you use a stainless steel pan however you will need to follow a few tips to prevent them from sticking. A non-stick pan is easier to use and will guarantee the scallops don't stick to the pan however it won't get quite as good a sear as the stainless steel.To use stainless steel - ensure the pan is very hot and the protein is as dry as possible. Do not try to flip the protein over too early - it will release from the pan once it's fully cooked on that side.