For the Stock: Adjust oven rack to middle position; heat oven to 450 degrees F. Place carrot, celery, onion, protobellos and garlic head on rimed baking sheet; drizzle with oil and toss to coat. Add tomato paste and toss again until evenly coated. Spread vegetables in even layers, set garlic head cut side up; roast until vegetables are well browned, 25-30 min.
Combined roasted vegetables, broth, leek greens, parsley, thyme, bay and porcini in a large heavy-bottomed dutch oven; cover and bring to simmer over medium high heat, then reduce heatto medium low and simmer partially covered for 30 min. Remove garlic head; using tongs, squeeze garlic head at root end until cloves slip out of their skins. Using fork, mash garlic to paste in small bowl and set aside. Strain stock through large fine mesh strainer, pressing on solids to extract as much liquid as possible; discard solids in strainer.
For the Soup: Rinse out and wipe dutch oven; add tomatoes, potatoes, carrots, celeriac, reserved whites of leeks, strained broth, and garlic paste to pot and bring to simmer ofer medium high heat, then reduce heat to medium low and simmer partially covered until vegetables are tender when poked with skewer or paring knife about 25 min. With back of wooden spoon mash some potatoes against the side of the pot to thicken soup. Stir in escarole and lima beans (if using): cook until escarole is wilted and limas are heated through, about 5 min. Stir in parsley; adjust seasonings with salt and pepper. Serve immediately with Crostini floated on soup, if desired
For Crostini: adjust oven rack to middle position; heat oven to 400 degrees. Place baguette slices on baking sheet; bake until dry and crisp, about 10 min, flipping slices halfway through baking time. While still hot, rub one side of each slice with raw garlic clove, then drizzle lightly with olive oil and sprinkle with salt and pepper to taste.