Muddle the agave and half of Fresno slices in a cocktail shaker. Fill the shaker with ice and add the tequila, mango juice, passionfruit juice, orange liqueur and lime juice. Shake vigorously to combine. Strain into a glass over a large ice cube. Garnish with a Fresno slice.
For fresh mango juice, peel mango and place the flesh in a blender. Add a splash of lemon or lime juice and blend until smooth. Strain the juice through a fine-mesh sieve. For fresh passionfruit juice, cut ripe passionfruit in half and scoop out the flesh. Press the flesh through a fine-mesh sieve set over a bowl. Use the strained liquid for the juice; reserve the seeds for garnish.