Preheat your wok or large skillet over medium-high heat until shimmering, and add the oil, swirling to coat the surface.
Add the thin slices of garlic to the hot oil, stirring constantly. Cook for about 30 seconds until fragrant and lightly golden, but not browned.
Quickly add the fresh spinach to the pan, a handful at a time if necessary. Use tongs to turn and coat the leaves in the garlic-infused oil, watching them wilt immediately—this should take about 1-2 minutes.
Sprinkle a pinch of salt and a pinch of red pepper flakes over the greens, tossing gently to combine and distribute the seasonings evenly. Continue cooking until the spinach is glossy and tender, about 2 more minutes.
Squeeze the fresh lemon juice over the greens, tossing gently to brighten the flavors. Turn off the heat and give everything a final stir.
Transfer the spinach and garlic stir fry to a serving plate, allowing it to rest for a moment to settle the flavors. Serve immediately while hot, enjoying the fragrant, glossy greens with tender garlic slices.