Pat the scallops completely dry with paper towels. Season both sides with salt and white pepper and set aside.
In a large saucepan, melt 2 tablespoons of butter over medium heat. Add the minced shallots and sliced leeks and sauté for 4 to 5 minutes until completely softened. Add the garlic and cook for another 30 seconds until fragrant.
Pour in the entire bottle of champagne. Bring to a boil and reduce for about 10 minutes until the champagne has decreased by half.
Add the seafood stock and the pinch of saffron. Stir to combine and simmer for 15 minutes.
Strain the sauce through a fine mesh sieve and return the liquid to the pot. Discard the solids.
Stir in the heavy cream slowly over low heat. Add the lemon juice and taste and adjust salt and white pepper as needed. Keep warm over the lowest heat.
In a separate skillet, melt the remaining tablespoon of butter over high heat until foaming. Add the scallops in a single layer and sear for 90 seconds per side without moving until deeply golden on both sides.
Ladle the champagne saffron sauce into shallow bowls. Place the seared scallops in the center. Top with a small spoonful of caviar, fresh tarragon leaves, and a pinch of micro greens. Serve immediately.