For the red peppers, turn on the oven's broiler. Line a baking sheet with foil and lighly spray or brush it with olive oil. Slice the red peppers in half from top to bottom, discarding the stem and seeds. Place the red pepper halves, skins facing upwards, on the baking sheet. Place the baking sheet under the broil and broil the peppers until the skins are thoroughly blackened on the top. Remove the baking sheet and, using kitchen mitts to protect your hands, enclose the roasted peppers in the foil on which you've baked them, pinching the foil closed. (Alternatively, you can transfer the red peppers to a plastic bag for a few minutes. Wrapping them allows them to steam and will make peeling them a cinch.) Allow the enclosed peppers to rest at room temperature until cool enough to handle. When cool, gently remove the blackened peels and slice the peppers into 1inch strips.)

In a small jar or bowl, layer the pepper slices, sliced basil, and garlic so that they are tightly packed. Drizzle olive oil intermitently between the layers and cover the final layer of peppers with about 1/4 inch of oil. Store, covered, in the refrigerator for up to 2 weeks.

Place the lentils in a pot and fill with water to cover by at least 2 inches. Put the pot over high heat and bring to a boil. Reduce to a simmer and cook until the lentils are tender, about 25 minutes.

Meanwhile, cut the onions into very thin slices and combine with the red wine vinegar. Let the onions soak in the vinegar, stirring occasionally, until the onions soften, about 10 minutes. Drain the vinegar.

When the lentils are tender, drain them well and, while still hot, pour them over the onions. Stir to combine and leave at room temperature to cool. When the lentils have cooled slightly, add the marinated red peppers, garlic, capers, rosemary, olive oil, and salt and pepper. Stir to combine. Serve immediately or chill until ready to serve (bring to room temperature before serving). Just before serving, cut the lemon into wedges and squeeze the juice liberally over the salad.