Heat the grill to high; if using charcoal, wait until the coals are ashed over and glowing.
Brush the peppers with canola oil, season with salt and pepper and grill, covered, turning occasionally, until charred on all sides and tender, about 10 minutes. Transfer to a bowl, cover tightly with plastic wrap, and let steam and soften for 15 minutes. Keep the grill at high heat.
While the peppers steam, smash the garlic cloves, sprinkle with salt, and use the blade of your knife to work the garlic to a puree. Transfer to a bowl and add the olive oil, vinegar, basil, parsley and salt and pepper. Peel, seed and dice the peppers and add them to the bowl. Taste and season as needed. Let the relish sit at room temperature while you cook the broccoli rabe.
Bring a large pot of salted water to a boil. Add the broccoli rabe, cook for 2 minutes, then immediately plunge it into an ice water bath. Drain until almost dry but still slightly damp (a little bit of water helps the rabe steam on the grill). Brush the rabe with canola oil and season with salt and pepper. Grill the broccoli rabe until crisp-tender and slightly charred on both sides, about 1 minute per side. Transfer to a platter, top with the pepper relish and drizzle with a bit of olive oil.