Preheat the oven to 325ºF. Line a baking sheet with parchment paper. In a bowl, combine the walnuts, cashews, oats, and flaxseed. Add the honey, coconut oil, vanilla, cinnamon, and salt. Stir to coat. Arrange in an even layer on the baking sheet and bake for about 40 minutes, stirring every 15 minutes, until golden brown. Cool completely on the baking sheet.
In a bowl, combine the yogurt, honey, lemon juice, and lemon zest. Layer the yogurt with the granola and berries in glasses or small bowls. Garnish with mint and serve.