Step 1Cook the bacon in a skillet over medium-low heat, rendering the fat. Strain the bacon fat into a measuring cup and, if you are hungry, eat the bacon. Add enough oil to the bacon fat to make two-thirds cup.
Step 2In a blender, blend the mustard, vinegar and salt. With the blender running, slowly pour in the fat. It will create a thick emulsion. Scrape down the sides of the blender container and pulse again to combine. Set aside until ready to use. This vinaigrette can be refrigerated in a small, tightly covered jar for several days, but it will solidify. Before using, warm it gently to return it to a liquid state and shake it very well to re-emulsify.
Step 1Steam the potatoes until easily pierced with a fork, about 12 minutes. Transfer them to a bowl and sprinkle with the vinegar. When cool, cut them in half, if very small, or in quarters.
Step 2Trim the base of the roots of the onions, leaving a small amount of base intact to hold the onion together. Trim the green tops to make 3- to 4-inch pieces. Steam the onions until just tender, about 3 minutes. Remove from heat and cool slightly. Remove any tough outer skin and cut the onions lengthwise in quarters. Add to the potatoes.
Step 3Trim the roots of the radishes and trim the tops to within one-half inch of the radish. Steam the radishes just until the color and texture have barely softened, about 1 minute. Transfer the radishes to a separate bowl (their color will bleed if they are added directly to the potatoes). When cool, cut in quarters.
Step 4Cut the fish sections in half crosswise, on a slight bias. Season lightly with salt on both sides. Place the flour in a bowl and dredge the fish in the flour to lightly coat on both sides. Pat lightly to remove any excess.
Step 5Heat the oil in a large non-stick skillet over medium-high heat. When it is very hot, add half of the fish, being careful not to overcrowd the skillet (if necessary, cook the fish in three batches rather than two). When the fish is well browned on one side, about 5 minutes, turn and finish cooking on the other side, another 5 minutes.
Step 6While the fish is cooking, assemble the salad. Add the radishes to the potatoes and onions and drizzle with about 2 tablespoons of the vinaigrette. Toss to coat lightly, then sprinkle with salt and some of the minced parsley, chives and tarragon. Taste and adjust seasoning.
Step 7Arrange the salad in the center of 6 to 8 plates. As the fish is done, arrange it around the outside of the salad, propped up against it. Season the fish lightly with a drizzle of the remaining vinaigrette and a little of the minced herbs and serve immediately. Remove the fish bones before eating.