Make vinaigrette: heat the olive oil in a large saute pan and saute the pancetta, garlic and shallot over medium-high heat for 1 minute. Remove from the heat and stir in the vinegar and half of the basil. Add the onions and return to the heat for about 30 seconds. Remove from the heat and stir in the remaining basil.
Core the tomatoes; cut a slice off the tops and bottoms and discard. Cut each tomato into 3 thick slices. Try and keep the slices in order!
To assemble: place a bottom tomato slice on each serving plate. Place a layer of the greens on top of the tomato, using up about half the greens. Place a layer of the onions and a little of the vinaigrette over the greens, and then the center tomato slice. Add another layer of the greens (use the rest), onions (save some for garnish) and top with the last slice of tomato. Garnish with onion and a basil sprig.