Place the wheat berries and 3 cups of boiling salted water in a saucepan over low heat and cook, uncovered, until they are soft, about 45 minutes. Drain.
Place the squash, onion, red and yellow bell peppers and green pepper on a sheet tray and drizzle with olive oil. Sprinkle with salt and pepper and toss to combine. Lay out a piece of foil and add the garlic. Drizzle with oil and sprinkle with salt and pepper. Wrap the garlic in the foil and add to the sheet tray. Roast the vegetables until the peppers are lightly charred and the squash is tender and evenly browned, about 30 minutes, tossing halfway through for even cooking.
Remove the vegetables from the oven and let stand until cool enough to handle. Finely dice the onion and peppers. Add to a large bowl with the squash and set aside.
Add the garlic to a small bowl and mash it with a fork Stir in the vinegar and mustard and season with salt and pepper. Whisk in the olive oil.
In a large bowl, combine the warm wheat berries, roasted vegetables and vinaigrette. Allow the salad to sit for at least 30 minutes for the wheat berries to absorb the dressing. Just before serving, add the currants and basil and season with salt and pepper. Serve at room temperature.