Melt half the butter in a large frying pan, add shallots and bacon and sauté over medium-high heat until shallots are soft (3-5 minutes). Add cabbage and cook, stirring occasionally, until starting to wilt (3-5 minutes), season to taste with sea salt and freshly ground black pepper. Increase heat to high, add wine and place fish skin-side down on top of cabbage, scatter with thyme, season to taste and cover with lid. Reduce heat to medium and cook until fish is cooked through (5-7 minutes).
Meanwhile, boil potatoes until tender (7-10 minutes). Drain, add remaining butter and parsley, toss to combine and season to taste. Serve trevalla with cabbage, potatoes and lemon.