Preheat the oven to 400°F (200°C). Line an extra-large baking sheet (15 by 21 inches/38 by 53 cm) with parchment paper.
Make the potatoes Place the potatoes on the baking sheet and toss with the olive oil until thoroughly coated. Spread the potatoes into an even layer. Season with a couple of pinches of salt and pepper.
Make the roasted garlic Slice the top off the garlic bulb so all the individual garlic cloves are trimmed. Place garlic bulb on a square of aluminum foil (about 8 inches/20 cm square) and drizzle the top of the cloves with the olive oil. Wrap the garlic bulb entirely in the foil and place it on the baking sheet with the potatoes.
Roast the potatoes and garlic for 20 minutes, then remove pan from the oven and flip the potatoes with a spatula. Return the potatoes and garlic to the oven and continue roasting for 15 to 20 minutes more, until the potatoes are golden and fork-tender.
Make the pesto In a food processor, combine the pesto ingredients and process until mostly smooth, stopping to scrape down the bowl as necessary. Keep the pesto in the processor because we will add the roasted garlic as the final step.
Remove the potatoes and garlic from the oven. Carefully unwrap the garlic bulb and let cool for 5 to 10 minutes, until it’s cool enough to handle.
Turn off the oven and return the potatoes to the oven with the door ajar so they stay warm. (You can also put the potatoes into an oven-safe casserole dish so the dish stays warm when serving.) Squeeze the roasted garlic cloves out of the bulb. You should have about 2 packed tablespoons (30 mL) of roasted garlic. Add it into the food processor with the pesto. Process until mostly smooth—you can add a touch more oil if necessary to get it going.
Assemble the salad This is the important part where you need to act fast; I like to assemble the salad very quickly so that it’s warm when I serve it. Grab a large serving bowl and place the arugula in the bottom of the bowl. You can break it up into smaller pieces with your hands a bit. Then, remove the potatoes from the oven and quickly place them into the serving bowl on top of the arugula. Toss the potatoes and arugula with the pesto until thoroughly combined. Taste and season with salt and pepper. Sometimes I add another drizzle of lemon juice if I feel like the dish needs more acidity. Sprinkle on the hemp hearts and serve immediately.