Use the shredding attachment on a food processor or the largest holes on a box grater to grate the jicama. Mix the vegetable with a generous pinch of salt, then let it sit in a colander in the sink or over a bowl for 20 to 30 minutes. Rinse the jicama, then wring it as dry as you can manage in a towel. Transfer to a large bowl.
Put the sunflower seeds in a medium skillet over medium heat. Cook, shaking the pan occasionally, until they are slightly darker and fragrant, 3 to 5 minutes. Transfer to the bowl with the jicama. Put the mayonnaise, mustard, vinegar, oil, and 1 teaspoon tarragon in a small bowl and sprinkle with salt and pepper. Whisk to combine, then taste and adjust the seasoning.
Put the grapes, celery, onion, and remaining 2 teaspoons tarragon in the large bowl and drizzle on some of the dressing. Toss gently to coat, then taste and adjust the seasoning, adding more dressing if you like. To serve, put the lettuce leaves on a serving platter or in individual bowls and top with a heaping spoonful of salad.