1Prepare the Italian sofrito: heat up a pan to medium heat, add olive oil, onions an garlic. Saute for 10 minutes, until fragrant and starting to turn golden. Add carrots and celery, stir, and saute an additional 10 minutes, until carrots are softening a bit.
2Add fennel, salt, and pepper. Cook over moderate heat, stirring occasionally, 6-8 minutes, until fennel is lightly browned. Add vegetable stock, coconut milk, and thyme, reduce heat to medium-low, and cook, partially covered, until all vegetables are tender but not at all mushy, about 20 minutes.
3Approximately 10 minutes before fennel-tomato ragout is done cooking, pat fish dry and sprinkle with salt and pepper. Heat 2 Tablespoons of oil in a non-stick skilled over medium-high heat until hot but not smoking. Add fish and cook, turning once, about 6-8 minutes.
4Stir mustard into ragout just before serving, taste, and season with salt and pepper as needed.
5Serve fish over fennel-tomato ragout, sprinkled with chopped fennel fronds