1) Position an oven rack 12 to 15 cm from the grill element. Preheat the grill to high. Line a baking sheet with heavy-duty aluminium foil.
2) In a small bowl, gently toss the tomatoes, shallots, 1/2 tablespoon of olive oil and vinegar together. Spread out the mixture on the prepared baking sheet. Set the baking sheet on a rack under the grill.
3) Grill until the tomatoes begin to show a few black spots, about 4 minutes with oven door closed. Turn the mixture, using a spatula, and broil 3 minutes longer with the oven door closed. Remove from the oven and transfer the mixture with a slotted spoon to a small bowl; set the bowl aside until needed. Retain the grill heat on high.
4) While the tomatoes are grilling, combine the ricotta, blue cheese, pesto, 1/2 teaspoon salt, lemon juice and lemon peel in a medium bowl.
5) Arrange the baguette slices on a large baking sheet and brush with the remaining 2 tablespoons of olive oil. Put the baking sheet on the rack under the grill and grill until golden, about 1 minute or so per side, watching carefully. Remove from oven.
6) To assemble, season the tomato mixture with remaining 1/4 teaspoon salt and peppercorns, then stir in the basil. Spread some of the ricotta mixture over each crostini. Top with the tomato/onion mixture, then sprinkle with grated cheese, dividing all the ingredients equally among the slices.
Transfer to a serving plate and serve.
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