Salt the zucchini. Cut each zucchini in half crosswise, then slice lengthwise into quarters. Cut off the seedy portions and discard (or eat); cut wedges into lengthwise ¼ inch slices. Toss in a large bowl with salt and let sit for 7-8 minutes. Rinse off the salt from the zucchini and gently squeeze to drain off water.
Prepare the sauce. In a small bowl, whisk together the soy sauce, vegetarian oyster sauce, ketchup, and sugar.
Stir-fry the zucchini. Heat a wok or saute pan over high heat until it is nearly smoking, then add the oil and quickly swirl it around to coat the bottom. Immediately add the garlic and cook until it smells delicious but hasn’t started browning, about 20 seconds. Add the zucchini and stir-fry to coat everything in the fragrant oil, about 1 minute. Add the sauce and stir-fry until the zucchini is softened and the sauce is slightly reduced, about 2 minutes. Plate with the tempura squash blossom, garnish with sesame seeds and serve.
