Place the watercress in a large heatproof bowl and set aside.
Preheat the grill. Place a large saute pan over medium-high heat.
Add the olive oil and when hot, add the bacon. Cook, stirring often, until most of the fat has been released and the bacon is crispy, about 6 minutes. Add the shallots and garlic, and cook for 1 minute. Add the walnuts and cook until just toasted, stirring, about 1 minute. Add the apples and honey and cook, stirring occasionally, for 3 minutes.
Season the blue fish filets on both sides with the Essence, and 1 teaspoon of salt. Place on the grill and cook for 3 1/2 to 4 minutes per side.
Season the bacon apple mixture with 1/2 teaspoon of salt, 1/2 teaspoon of the black pepper, and the basil. Add the vinegar and reduce by half, about 30 seconds. Remove the dressing from the heat and pour over the watercress.
Season the watercress with the remaining 1 teaspoon of salt, and 1/2 teaspoon of black pepper. Toss well to coat and divide the watercress among 4 plates. Place the fish on top of the watercress, and garnish with the fresh basil sprigs. Serve immediately.