In a large pot, stew the carrots, onion, butter and thyme (5-10 min).
Season to taste with salt, pepper and sugar.
Add the chicken stock and rice, bring to a boil, then reduce to a simmer until rice is cooked through.
Strain, but reserve the liquid.
Blend the solids in batches until smooth, adding the reserved liquid to reach desired consistency.
Before serving, make a compound butter by combining 5 oz softened butter with the minced mint. Shape as desired and return to the fridge to solidify.
Serve each bowl of soup with a slice of the mint compound butter.