Roast Garlic
Set the oven rack to the center position. Preheat your oven to 400 degrees F (200 degrees C).
Place the garlic in a center of a piece of foil that's large enough to completely wrap the garlic.
Cut the top of the garlic off with a sharp knife to expose the cloves. Generously drizzle olive oil over the cut garlic cloves, sprinkle with the ¼ teaspoon kosher salt, then tightly wrap the head of garlic in the foil and place on a baking sheet or in a baking dish.
Roast the garlic in your preheated oven for 45 minutes. The garlic cloves will be soft enough to easily pierce with a fork, golden brown and extremely fragrant. Let cool enough to touch.
Squeeze the garlic cloves from their paper skin into a small bowl or container. Mash half of the cloves with a fork and set aside.
Mashed Potatoes
Peel the russet potatoes and cut them into smaller, equal-sized pieces.
In a large pot, add the potatoes and pour in enough water to cover them over an inch above the top of the potatoes. Add the tablespoon of chicken bouillon and bring the pot of water to a boil. Boil until the potatoes are fork-tender, about 20-30 minutes. Drain the potatoes in a colander when done.
Press the potatoes through a potato ricer or food mill back into the pot or into a large serving bowl.
Melt the unsalted butter in a saucepan over low heat with a sprig of rosemary. Keep the melted butter warm over that low temperature (don't burn the butter, but browning the butter is never a bad thing!) for up to 15 minutes.
Remove the rosemary sprig from the butter then stir most of unsalted butter into the potatoes, reserving the rest for serving. Gently fold and mix in the roasted garlic and heavy cream with a wooden spoon until well combined.
Taste the mashed potatoes and season with salt and pepper as needed.
Pour the rest of the butter over the mashed potatoes and garnish with the finely chopped rosemary.
Serve warm to hot.