Stir flour and ground flax seeds together in medium bowl. Whisk in milk and soy sauce. Allow to sit for about 10 minutes, adding a little more milk if it seems too thick to evenly coat cauliflower.
Preheat oven to 400°F and lightly oil a baking sheet.
Stir panko breadcrumbs, black pepper, and cayenne together in medium bowl.
Dip each cauliflower floret into flour-milk batter and roll to coat, shaking off any excess batter. Transfer to panko breadcrumb mixture and gently roll to coat. Place on prepared baking sheet. Lightly spray with oil using a mister or cooking spray.
Bake until fork-tender and lightly browned, 20-25 minutes.
Manchurian Gravy
While cauliflower bakes, place oil in medium saucepan over medium heat. Add white parts of scallions, garlic, ginger, celery and serrano pepper. Sauté until fragrant, about 1 minute. Add broth, soy sauce, tomato paste, agave or maple syrup, rice vinegar, chili paste and black pepper. Allow to simmer for 10 minutes, stirring occasionally.
Stir chilled water and cornstarch together in small bowl. Add to gravy mixture and stir to incorporate. Allow to simmer for about another minute, just until mixture thickens. Stir in scallion greens and cilantro.
To Serve
Divide cauliflower onto plates or bowls and top with gravy. Serve immediately.
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