Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper and set aside.
Place the cauliflower florets in a large bowl, and toss with 3 tablespoons of avocado oil until lightly coated. Sprinkle with the cornstarch, and toss until coated. Season with salt if desired.
Heat 1/4 cup avocado oil in a large skillet over medium-high heat. Add the cauliflower florets to the skillet, spread them out in an even layer, and cook, turning just once or twice, until they start to brown. This should take about 10 minutes. Keep an eye on the heat, and decrease to medium if needed. Transfer the florets to the lined baking sheet, and bake for 10 to 15 minutes until crispy on the outside and tender on the inside.
Meanwhile, prepare the sauce. In a small saucepan, whisk together the apple cider vinegar, ketchup, maple syrup, soy sauce, and garlic. Bring to a boil. In a separate bowl, stir together the cornstarch and cold water. Add this to the saucepan, stir, and bring back to a boil. Cook for a few minutes, stirring frequently, until thickened. You may need to add another round of cornstarch and water. Remove from heat.
When the cauliflower is ready, toss in your desired amount of sauce. Serve over rice, and garnish with scallions and sesame seeds. I recommend serving with a source of protein, like edamame or tofu, for a complete meal.