Preheat oven to 375 degrees F.
Cut pumpkin in half and deseed, rubbing clean flesh with 1 tablespoon olive oil. Rub garlic and thyme with a 1/2 tablespoon oil, then place them on the baking sheet and cover them with one pumpkin halve. Place the other pumpkin halve cut side down next to it. (You probably won't need this half because you should get enough meat from the one with the garlic, etc.) Roast for 40 minutes, or until flesh of pumpkin is completely soft.
Let cool, so you can handle, then scrape flesh out of the halve that was covering the garlic & thyme. Measure out 1 1/4 cups flesh, then add to a food processor.
Remove papery skin from garlic and discard, and also take thyme leaves off the sprigs. Add garlic & thyme leaves, plus remaining 1/2 tablespoon oil to food processor and blend all until smooth (add a tablespoon of water if needed to become smooth).
While pumpkin is roasting, place potatoes and 1 teaspoon salt in large saucepan; cover with cold water. Bring to boil, reduce heat to medium, and simmer until tender, about 25 minutes. Drain well, reserving 1 cup of the cooking water.
Then return potatoes to to pot. Mash potatoes until somewhat smooth, then stir butter into them.
Add warm milk and continue lightly mashing until absorbed. Add 1/2 cup of cooking water, and incorporate. (You can add more if you like them smoother, and continue mashing. The texture part of this recipe is all up to you!) Season with remaining 1 teaspoon salt, garlic powder and black pepper.
Using a spoon, add pumpkin mixture at once to mashed potatoes and fold in, creating swirls throughout. Serve warm.