Preheat the oven to 350F/180C.
Slice the tip off the garlic to expose the cloves. Drizzle with olive oil, wrap in foil or baking paper, then pop in the oven for 45-60mins, or until deep golden, caramelized and sticky (you can peek to check). Remove from the oven and leave to cool slightly, then turn the temp up to 400F/200C.
Meanwhile, place the potatoes in a pot of cold, heavily salted water and bring to a boil. Cook until knife tender (15mins or so), then drain and leave in the colander for 1 minute to let some steam escape. Afterwards, place them back in the pot. Squeeze the garlic out of the cloves into the potato, then add the cream and butter. Mash until smooth.
Mash in 1 1/2 cups / 150g of the cheddar, then generously season with salt and pepper. I typically end up with 3/4 tsp salt and 1/4 tsp pepper, but work to taste. Make sure the potato is well-seasoned (nobody likes bland mashed potatoes).
Scrape the mashed potato into a baking dish, then scatter over the remaining cheddar. Place in the oven for 20 minutes, or until golden and bubbly. I like to flick on the broiler/grill to brown the top, but that's optional. It might be a little loose fresh out of the oven, but it firms up more as it rests.
Sprinkle over a handful of finely diced chives, then serve and enjoy!