Line a baking sheet with parchment paper. Place the frozen croissants on the baking sheet. Allow them to thaw slightly at room temperature for 30 minutes.
Set a rack in the center of the oven. Preheat the oven to 375 degrees F.
Spread the nonpareils onto a small rimmed plate. Lightly pat the top of the croissants dry (they will leach water as they thaw). One at a time, lightly brush some of the egg white on top of the croissants then dip them into the nonpareils, pressing lightly so that they adhere. Place them back on the baking sheet, spacing them evenly apart (they will double in size as they bake).
Bake until the croissants are golden brown, puffed and cooked through, about 25 minutes, rotating the baking sheet once halfway through the cooking time.
Transfer the croissants to a wire rack to cool completely, about 20 minutes.
Meanwhile, combine the confectioners' sugar, mascarpone and jam in a large bowl. Use an electric mixer to blend until completely smooth. Add the heavy cream and whip on low speed until combined, then gradually increase the speed and whip until medium-stiff peaks form. Transfer the strawberry cream to a piping bag fitted with a small round tip about 1/4-inch wide.
With a paring knife, make a small ''X'' a little off-center on the underside of a cooled croissant. Insert the tip of the piping bag into the center of the ''X'' and fill with the strawberry cream. Repeat with the remaining strawberry cream and croissants. Serve immediately.