Wash the strawberries and remove stems. Add 2 fresh strawberries to the bottom of a champagne flute and "muddle" (gently smash into the bottom of the glass).
Pour in Champagne or Prosecco until the glass is about 2/3 full. Top with desired amount of pineapple juice.
Garnish with a fresh slice of pineapple or strawberry.
Amounts as listed are only estimates. Feel free to adjust as desired. You can make mimosas a little in advance, but I recommend not making them more than an hour ahead of you will lose some of the carbonation from the champagne.