Trim the base of the maitake mushrooms and tear them into small pieces. Separate the stems and leaves of the chrysanthemum greens; roughly chop the stems and finely chop the leaves.

Place the oysters flat-side down in a steamer and steam for about 3 minutes. Flip them over and steam for an additional 6 minutes. Remove the oysters and separate the meat from the shells, reserving the steaming liquid.

Heat olive oil and garlic in a frying pan, then sauté the maitake mushrooms. Add the reserved oyster steaming liquid and the seasoning base (dashi and mirin). Simmer briefly, then season with soy sauce and salt. Thicken with the cornstarch mixture.

Cook the spaghettini in salted boiling water for 2 minutes less than the package instructions.

Add the cooked spaghettini to the frying pan with the sauce and toss to coat. Add the steamed oysters and chrysanthemum stems, mixing well. Turn off the heat and fold in the chrysanthemum leaves.