For the marinade, combine lemon juice, mustard, soy sauce, honey, garlic power, 3 tablespoons warm water and 1/4 teaspoon of the pepper in a small bowl with a whisk to dissolve mustard and honey completely. Set aside.

Preheat the oven to 375°F.

Slice tofu crosswise into 8 even slices and place in one layer in a foil lined baking dish.

Pour about half of marinade over tofu, and reserve the other half.

Marinate tofu for 30 minutes, turning it over halfway through.

Bake tofu until all liquid has disappeared and tofu has begun to brown, about 45 minutes.

Meanwhile, in a medium bowl, toss the mushrooms with a tablespoon of the oil and sprinkle with salt and remaining 1/4 teaspoon pepper.

Transfer to a baking dish and roast until mushrooms are light brown and tender, 10 to 15 minutes.

To make salad dressing, whisk the remaining 1/4 cup oil into reserved marinade and season with salt and pepper, to taste.

Place a handful of spinach onto each plate, top with one or two pieces of tofu and sprinkle with roasted mushrooms.

Drizzle each salad with some of the dressing.