Cut the fillets into bite-sized pieces; 2? cubes would be a good size. Season with salt and pepper.
Trim the ends and sides of the chicharo. Peel the carrot, cut into two lengthwise then slice thinly.
Slice the mushrooms (caps and stems).
Mix together the chili bean sauce, oyster sauce, tapioca starch, sesame seed oil, sugar and water. Stir until the starch and sugar has completely dissolved.
Heat the wok. Pour in about 2 tablespoonfuls of cooking oil. Add the carrots and stir fry over high heat for about 15 seconds. Add the chicharo and cook, stirring, for another 15 seconds. Add the sliced mushrooms and continue cooking for another 30 to 45 seconds or just until the mushrooms are cooked. Transfer the vegetables to a dish.
Pour in 2-3 tablespoonfuls of cooking oil into the hot wok. Saute the ginger, garlic and onion. Add the seasoned yellow fin tuna fillets and cook over high heat just until the fish changes color. Return the vegetables to the wok. Make a well in the center and pour in the sauce. Stir until it starts to thicken. Toss the fish and vegetables with the sauce and cook for another 10-15 seconds.
Serve at once.