In a large saucepan, heat oil on medium-low. Add leeks and cook, stirring occasionally, for 5 minutes. Add garlic and cook, stirring frequently, for 1 minute. Add potato, cauliflower, broth, 1 cup water and pepper and increase heat to bring to a boil; reduce heat to low, cover and simmer until potatoes and cauliflower are very tender, 20 minutes. Remove from heat.
Meanwhile, mist a medium skillet with cooking spray and heat on medium-high. Add bacon and cook, turning once, until golden brown and cooked through to desired crispness. Transfer to a paper towel–lined plate to drain. Chop and set aside.
With an immersion blender, purée soup until smooth. (Alternatively, carefully transfer soup to a blender, in batches if necessary, remove plastic stopper from lid, cover with a kitchen towel and purée until smooth; return to saucepan.) Place saucepan on low and stir in 1/4 cup cheese until melted. Season with additional pepper, if desired. Ladle soup into serving bowls and top with remaining 1/4 cup cheese, bacon and scallions, dividing evenly.