For marinade: Whisk all ingredients together.
For mushrooms & sauce: Rub marinade over mushrooms so they are well coated and let sit at room temperature for 30 min. Heat 1 Tbsp. of the oil in a large skillet over medium-high heat. Add mushrooms and cook for 3 to 5 min. per side until slightly charred but not too soft. Transfer to a plate to rest while you prepare sauce. Reduce heat to medium and add remaining 1 Tbsp. oil to the skillet. Add onions and cook, stirring frequently, for 2 to 3 min. Whisk together soy sauce, honey, garlic, and ginger; stir mixture into onions. Bring sauce to a boil then reduce heat to low. Whisk water and cornstarch together until smooth then stir into sauce. Cook until sauce thickens, 2 to 3 more min. Slice mushrooms into strips and stir into sauce. Cook just until mushrooms are warmed through. Serve topped with scallions over cauliflower “rice”.
For cauliflower: Place florets in a food processor and pulse until texture resembles rice or couscous, about 15 to 20 pulses. (Alternatively, grate cauliflower into a bowl using a hand grater.) Heat coconut oil in a large skillet over medium heat. Add onions, garlic, and sesame seeds. Saute until fragrant, 3 to 4 min. Stir in “riced” cauliflower, rice vinegar, sesame oil, and salt & pepper to taste. Cook for 6 to 8 min. until cauliflower is cooked through but the texture is still firm.