Make toasts: Trim and discard crusts from bread. Spread 1 side of each slice evenly with butter and season with salt and pepper. Halve slices diagonally to form triangles and arrange, buttered sides up, on a baking sheet. Toast bread, turning once, in middle of oven until golden brown, about 4 minutes. Transfer toasts to a rack to cool.
Make soup: Put squash, cut sides down, in 2 greased shallow baking pans and roast in upper and lower thirds of oven, switching position of pans halfway through roasting, until very tender, about 1 1/4 hours. Cool and scoop out flesh.
Purée squash in batches in a blender with jalapeño, cumin, and broth until very smooth (add additional water as needed to facilitate puréeing).Transfer to a 6- to 8-quart heavy pot and stir in 5 cups water. Simmer, stirring, 10 minutes, then stir in lemon juice, enough water to thin to desired consistency, and salt and pepper to taste.
Serve soup in bowls topped with slices of toast spread with pesto.
Cooks' notes: Toasts may be made 2 days ahead, cooled completely, and kept between layers of paper towels or wax paper in an airtight container at room temperature.
Squash purée may be made 2 days ahead. Cool completely, then chill, covered.