Over medium-high heat, in medium saute pan, heat the butter and sage leaves until the butter becomes a dark brown color, verging on being burnt. Remove from heat and discard sage leaves. Let cool slightly. Whisk together the salt, pepper, red wine vinegar, balsamic vinegar and cooked sage butter in a medium mixing bowl. Slowly whisk in 1/4 cup of grapeseed oil until well blended.
Arrange the sliced tomatoes and mozzarella cheese, on a serving dish, in circular design alternating between the tomatoes and cheese, leaving a portion of each slice exposed. Drizzle with remaining 2 tablespoons of grapeseed oil, the raspberries, gooseberries, and feta cheese. Evenly pour the vinaigrette over the salad. Garnish with mint leaves and serve.