For the pickled shallots: In a glass jar, combine all ingredients. Let stand at least 30 minutes at room temperature, or up to 2 days in the fridge.
For the dressing: In a glass jar, combine all ingredients. Keep at room temperature until serving, or up to 2 days in the fridge.
For the salad: Preheat the barbecue or a grill pan over medium-high heat. Cut the Bergeron Le Populaire cheese into 1 cm slices, brush with oil, and grill 1 minute per side to lightly brown.
In a large serving dish, arrange the watermelon and cucumber. Cut the cheese into bite-size pieces and add to the salad. Drizzle with dressing, top with pickled shallots and pistachios, garnish with fresh mint, and serve.