Drizzle 1 tablespoon olive oil in a Dutch oven or baking dish large enough to hold the cauliflower; set aside.
Peel off the outer leaves of the cauliflower and trim the core just enough for the cauliflower to sit flat.
Place the remaining olive oil in a small bowl and use a microplane grater to grate the garlic cloves into it.
Add the herbs, salt, and pepper and whisk well.
Brush the mixture over the cauliflower, then press on the cheese.
Cover the Dutch oven or baking dish and roast for 40 minutes.
Carefully uncover and roast for another 15 minutes to lightly brown the crust.
While the cauliflower is roasting, make the pea pesto: Place the basil, peas, garlic, cheese, and walnuts in a food processor and pulse until very finely minced.
With the machine running, add the olive oil and process until smooth.
Add the water and pulse a few times. Season to taste with salt and pepper if needed.
To serve, cut the cauliflower into wedges, plate, and drizzle with the pea pesto and a few drops of olive oil.