Whisk the lemon juice and mustard in a large bowl. Gradually whisk in the olive oil until smooth. Add the tomatoes and sliced fennel and toss. Season with salt and pepper; set aside.
Preheat a grill to medium high and brush the grates with olive oil. Season the salmon on both sides with the ground fennel, salt and pepper. Brush the polenta rounds with olive oil. Grill the salmon and polenta until well marked and the salmon is just cooked through, 3 to 4 minutes per side. Meanwhile, grill the lemon halves cut-side down until marked, about 3 minutes.
Add the fennel fronds and basil to the fennel salad; toss. Serve the salmon with the polenta, fennel salad and grilled lemons.