Peel off tough outer skin of broccoli stem, then halve broccoli crosswise (top half with florets should equal length of bottom half). Cut bottom half (stem) lengthwise into 1/2-inch-wide wedges, then cut top half (stem with florets) into long spears with 1/2-inch-wide wedges.
Heat a wok over high heat until beginning to smoke, then add canola oil and swirl to coat sides. Stir-fry garlic until golden, about 30 seconds, then add broccoli and stir-fry 5 minutes. Stir in oyster sauce and stock and cook, covered, until broccoli is crisp-tender, about 3 minutes. Remove from heat and toss with sesame oil.
