Warm the apricot filling in a sauté pan over medium-low heat and then stir in the lemon juice and the sugar, to taste (the lemon and the sugar should enhance the apricot flavor and not overwhelm it). Remove pan from heat, strain mixture into a bowl and let cool completely.
Scoop 2-4 teaspoons (to taste) of apricot mixture into a Champagne flute and then fill the glass with Champagne/sparkling wine.
Carefully stir the cocktail until the apricot mixture is evenly distributed, and then serve immediately.
Notes
Prosecco is typically less than Champagne, so consider that when you're picking out your bubbly!