Preheat oven to 400˚F.
Slice off the top of the head of garlic, exposing the cloves.
Place leeks, cauliflower, broccoli, zucchini, potatoes, and garlic on a large sheet pan. Drizzle with olive oil, 1 tsp salt, and pepper; toss gently.
Roast vegetables for 30 minutes, until fork tender.
About 10 minutes before vegetables finish roasting, heat broth and peas in a small pot until simmering.
Add half the garlic cloves and all roasted vegetables to a blender with hot broth and peas, lemon juice, spinach, parsley, remaining salt and red pepper flakes; blend until smooth.
To make grilled cheese croutons, melt butter in a pan over medium low heat.
Toast one bread slice until golden brown; flip and add 2 cheese slices on caramelized side.
Spread pesto on cheese, then top with remaining 2 cheese slices.
Place other bread slice cooked side down on top of cheese.
Cover with lid and cook until bottom is brown and crispy, then flip and cook until other side is brown and cheese is melted.
Remove and slice into small cubes.
Serve soup hot, topped with pesto grilled cheese croutons if desired.