Pasta Salad with Roasted Broccoli

2 lb. (2 heads) broccoli
3 garlic cloves, halved lengthwise (skin on)
2 tbsp. olive oil
Coarse salt and ground pepper
8 oz. penne pasta
1 to 2 tbsp. fresh lemon juice
8 oz. jarred roasted red peppers, rinsed and cut into 1/2-inch strips (1 c.)
1/4 c. pitted Kalamata olives, halved lengthwise
1/2 c. walnuts, broken into pieces

Preheat oven to 450°F. Trim 1 inch from stems of broccoli. Peel outer layer of stems; cut diagonally into 1/2-inch slices. Separate florets into bite-size pieces. On one large or two small rimmed baking sheets, toss broccoli, garlic, and 1 tbsp. oil; season with salt and pepper. Spread in a single layer; roast, rotating sheets and turning broccoli halfway through, until tender and beginning to brown, 15 to 20 minutes. Set aside.

Meanwhile, cook pasta in a large pot of boiling salted water, according to package directions. Drain pasta, reserving 1/4 c. cooking water; rinse under cold water until cool, and drain. Return to pot.

Remove skin from roasted garlic. In a small bowl, mash garlic until it forms a paste; whisk in lemon juice and remaining tbsp. oil. Season with salt and pepper. Pour mixture over pasta; toss in broccoli, red peppers, olives, and walnuts. Add some reserved cooking water, if needed. Serve at room temperature or chilled.