Step 1: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
Step 2: In a large bowl, toss the cauliflower florets with olive oil, salt, and black pepper until evenly coated.
Step 3: Spread the seasoned cauliflower in a single layer on the prepared baking sheet. Roast for 20-25 minutes, or until tender-crisp and lightly browned, flipping halfway through. Set aside.
Step 4: While the cauliflower roasts, prepare the sweet and sour sauce. In a small bowl, whisk together the rice vinegar, soy sauce, brown sugar, and ketchup. In a separate small bowl, dissolve the cornstarch in water (or reserved pineapple juice) to create a slurry.
Step 5: Heat a large skillet or wok over medium heat. Add the minced garlic and grated ginger and sauté for 1 minute until fragrant. Be careful not to burn the garlic.
Step 6: Pour the prepared sweet and sour sauce mixture into the skillet. Bring to a gentle simmer, then slowly whisk in the cornstarch slurry. Cook, stirring constantly, for 2-3 minutes until the sauce thickens and becomes glossy.
Step 7: Add the roasted sweet sour cauliflower florets and drained pineapple chunks to the skillet with the sauce. Toss gently to ensure all the cauliflower and pineapple are thoroughly coated.
Step 8: Cook for another 2-3 minutes, allowing the sauce to meld with the cauliflower. Taste and adjust seasoning if necessary.
Step 9: Remove from heat. Garnish with thinly sliced green onions and sesame seeds before serving your delicious sweet sour cauliflower. Serve hot over rice or noodles.