Make the Sweet-Smoke Glaze: In a measuring cup or bowl, whisk together 1 teaspoon of the kosher salt, black pepper, ground mustard, honey, balsamic vinegar, olive oil, and liquid smoke until smooth. Set aside a few tablespoons of the glaze for brushing at the end.
Coat the Fish: Line a rimmed baking sheet with foil. Place the fish fillets on the sheet and spoon the remaining glaze over them, turning the fish to coat as evenly as you can.
Broil the Fish: Preheat the broiler on high and position a rack about 6 inches from the heat. Broil the fish until browned on top and it flakes easily, about 8 to 10 minutes, watching closely near the end since broilers run wild.
Start the Slaw Base: While the fish broils, heat the sesame oil in a saucepan or deep skillet over medium heat. Stir in the roasted garlic for about 30 seconds, just until fragrant.
Soften the Shallots: Add the chopped shallots and the remaining 1/2 teaspoon kosher salt. Cook, stirring, until the shallots soften, about 2 to 3 minutes.
Wilt the Slaw: Add the coleslaw mix and pour in the citrus rice vinegar. Toss to coat, cover with a lid, and cook until lightly wilted but still crisp, about 2 to 3 minutes.
Finish the Slaw: Transfer the warm slaw to a mixing bowl and toss in the roasted cashews.
Brush and Serve: Remove the fish from the oven and brush with the reserved glaze for extra shine and flavor. Serve the fish over the sesame cashew slaw, top with sliced green onions, and squeeze lemon over everything right before eating.