Preheat oven to 375º. Toss potatoes and beets with 2 T olive oil and a light sprinkle of salt and pepper. Place in a single layer on parchment-lined sheet pan and roast 15 min or until softened and beginning to brown. Meanwhile, gently toss maitake mushrooms in 1 T olive oil and pinch of salt. Add to pan with potatoes and beets for last 7 min of cook time.
While potatoes/beets/mushrooms are roasting, season halibut fillets with a sprinkle of salt and pepper and sear in 3 T olive oil in large, oven-proof skillet over medium-high heat for 3 min or until crust develops. Flip fish and transfer skillet to oven. Cook fish until flakey yet moist in center and internal temp reaches 130º, approx. 7-8 min. Immediately remove fish from hot pan and let rest to avoid overcooking.
Add butter and lemon juice to same hot skillet over medium-high heat and quickly reduce by half, approx. 30 sec. Add wine and reduce again, approx. 30 sec. Season sauce with a pinch of salt and pepper, to taste.
To plate, divide potatoes, beets and mushrooms among four plates. Add approx. 12 sprigs of cress to each, top with halibut, tomato wedges, lemon zest and sauce. Serve immediately with white wine, as desired.
* Other firm white fish, such as cod, could also be used for this recipe.
** To make this recipe keto, omit wine. To make this recipe paleo, omit wine and butter.