French · Cocktail
Equal parts Prosecco and orange juice, and the only two rules are that both are cold and the juice goes in first.

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The one thing that matters
Don't stir it. It mixes itself, and stirring knocks the bubbles out.
Warm Prosecco foams straight over the rim and you lose half the glass. Cold juice into a cold glass, then pour the Prosecco slowly down the side, and it behaves.
Don't use your best bottle. The orange juice flattens anything clever or delicate, which is exactly why an honest inexpensive Prosecco is the right call here and a good Champagne is a waste of money.
Fresh juice is better than carton, but carton is completely fine and anyone who tells you otherwise at eleven in the morning is being difficult. What matters far more is that it's properly cold.
Worth knowing: swap the orange for peach puree and it's a bellini. Grapefruit juice makes it sharper and considerably better if you don't like sweet drinks. Add a splash of orange liqueur and it becomes a buck's fizz with ambitions.
For a crowd, don't pre-mix. Put the juice in a jug, the bottles in ice, and let people build their own. A pre-mixed mimosa is flat within twenty minutes.