To make the strawberry simple syrup: In a medium saucepan over medium heat, combine the strawberries, orange peel, sugar, and water. Bring to a rolling simmer, then reduce to the heat medium-low and continue simmering, gently mashing up the strawberries with a wooden spoon, until the sugar has completely dissolved and the mixture thickens, about 20 minutes.
Remove from heat, stir in the sprigs of mint, and set aside and cool for 30 minutes. Once cool, discard the mint and orange peel, and strain through a fine-mesh strainer. Pour into an airtight container and store in the fridge until chilled, or up to 1 week. Stir well before using.
To make the mimosas: Stir the grand Marnier into the chilled simple syrup, mixing well to combine. Fill a champagne flute with 2 Tablespoons of the simple syrup mixture, 3 Tablespoons of the orange juice, and then fill almost all the way to the top with prosecco or champagne.
Place a small strawberry and slice of orange on the rim of each glass and garnish with a sprig of mint. Repeat with all glasses. (Makes about 6 mimosas).