Heat a medium saucepan over medium heat, add the butter and allow to melt. Add the shallots and Serrano pepper sauté until tender, about 5 minutes. Add the garlic and cook until fragrant, about an additional 30 seconds.
Add walnuts and white wine, cooking until the wine has reduced by half, about 5 minutes. Stir in the cilantro, heavy cream, lime juice, coriander and simmer for an additional 15 minutes. Use an immersion blender (or add to a blender) and pulse until the sauce is combined. Season to taste with salt.
For the crispy blackened steelhead:
Season filets with salt, pepper, chili powder, paprika and cayenne. Heat a large oven-proof skillet over medium heat, add the olive oil and heat through. Place the steelhead filets, skin-side up in the pan and cook until golden brown, about 5 minutes. Flip over and continue to cook until the filets are fully cooked through, about an additional 5 minutes.
For serving:
In a small saucepan heat the beans over medium-low heat.
Divide rice and beans among serving bowls and top with tomato and fish. Spoon the sauce over the top and serve.