Peel the jicama with a sharp knife, removing all the fibrous brown skin and the thin layer beneath. Cut into planks, then stack and slice into matchsticks about 3 inches long and 1/4 inch thick. Place in a large bowl of ice water while you prep everything else - this keeps it crisp and prevents browning.

Cut the top and bottom off each orange and grapefruit to create flat surfaces. Stand the fruit upright and slice away the peel and white pith in strips, following the curve of the fruit. Hold over a bowl to catch juice, cut between the membranes to release segments, and set segments aside. Squeeze the remaining membranes to extract any juice - you need about 2 tablespoons total reserved juice for the dressing.

Halve the cucumber lengthwise and scrape out the seeds with a spoon. Cut into thin half-moons. Thinly slice the red onion and soak in cold water for 5 minutes to mellow the bite, then drain well.

In a small bowl, whisk together lime juice, reserved citrus juice, olive oil, tajin, honey, and sea salt until emulsified. Taste and adjust - it should be bright, slightly spicy, and just sweet enough to balance the tart citrus.

Drain the jicama well and pat dry with a clean kitchen towel. In a large serving bowl, combine jicama, citrus segments, cucumber, red onion, and cilantro. Drizzle with dressing and toss gently with your hands or two large spoons - the citrus segments are delicate and will break if over-mixed.

Transfer to a serving platter or individual plates. Top with toasted pepitas, crumbled cotija if using, and avocado slices. Finish with a generous sprinkle of tajin over the top. Serve immediately while the jicama is still crisp and cold.