Drizzle 2 tablespoons olive oil in the bottom of a 4-quart rectangular baking dish (15" x 10" x 2" or larger).
Place all vegetables in a large bowl. Drizzle with remaining olive oil and toss gently to coat. Season with salt and pepper and gently toss to distribute.
Pour vegetables into prepared baking dish and spread into an even layer. Nestle head of garlic into vegetables, cut-side up and drizzle bare cloves with olive oil (recipe note #1).
Roast vegetables 40-60 minutes to your desired tenderness, gently tossing every 20 minutes. Squeeze roasted garlic cloves from their skins (be careful, they'll be hot!). Transfer vegetables and garlic cloves to serving bowl; season to taste with salt and freshly ground black pepper and garnish with fennel fronds if desired. Serve.
Notes
I don't mind my garlic cloves having a lightly toasted top. If you want them to be soft on top, wrap the garlic head in a little piece of foil and nestle into the vegetables.
Feel free to season with with herbs as well as salt and pepper - fresh thyme goes with just about everything!
To make a smaller batch, use about half of the amount called for in this recipe and cook in a single layer on a large rimmed baking sheet lined with parchment paper (you'll get more crispy edges this way too.)