For the polenta: Coat a 9- by 13-inch baking dish with a thin coat of olive oil.
Combine 4 cups water and 2 teaspoons salt in a medium saucepan and bring to a boil over high heat. Slowly add the polenta, whisking constantly. Reduce the heat to low, bringing the polenta to a simmer, and cook, stirring occasionally, until thick, 5 to 8 minutes. Turn off the heat and stir in the butter, cheese and rosemary. Pour the polenta into the prepared dish and cool completely until firm.
Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper or a nonstick silicone liner.
Unmold the polenta, cut into 1-inch squares and place on the prepared baking sheet. Bake until crisp and golden, about 1 hour.
For the salad: Meanwhile, whisk together the vinegar and Dijon in a small bowl along with some salt and pepper. Add the oil in a thin stream, whisking constantly. Toss the greens with the dressing in a large salad bowl. Add the tomatoes, cranberries, and Roquefort and toss again. Arrange the salad on a large serving platter or individual plates. Top with half the croutons and serve immediately. Reserve the remaining croutons for another use in an airtight container in the refrigerator for up to two days.