Preheat oven to 350˚F. Place the garlic, sliced sides up, in a small oven-safe dish. Drizzle with olive oil and sprinkle with salt and pepper. Cover with aluminum foil and bake for 1 hour 15 minutes. Remove from oven and allow to cool. Turn the oven up to 425˚F.
Place the broccoli in a large mixing bowl and drizzle with olive oil and sprinkle with salt and pepper. Toss to coat, and transfer to a large baking sheet - use 2 if needed to avoid over-crowding. Roast in the oven for 35 minutes, tossing occasionally. Add the almonds to the baking sheet and continue to cook another 10-12 minutes, until the broccoli is tender and the almonds begin to turn golden. Transfer to a large bowl and set aside.
Squeeze each clove of garlic from one of the heads into a medium bowl. Add both types of mustard, lemon juice, and a pinch of salt and pepper, and stir to combine. While whisking, pour in 5 tablespoons of olive oil. Taste, and adjust seasoning if needed. Pour the dressing over the roasted broccoli and almonds, and toss well to coat. Squeeze the cloves of garlic from the remaining head of garlic, trying to keep them whole, and use them to garnish the roasted broccoli. Serve immediately.