Toss 2 cups halved strawberries and 1 sliced shallot with 3 tablespoons sherry vinegar, 2 teaspoons sugar and a pinch of salt; let sit 10 minutes. Remove the berries and shallot with a slotted spoon. Whisk 2 tablespoons olive oil, 1 teaspoon dijon mustard, and salt and pepper into the vinegar mixture. Toss with 6 cups mesclun greens and the berry mixture. Top with hazelnuts and goat cheese.