In a large pot, heat 1 tbsp. oil over medium-high. Add the shrimp shells; stir until pink, about 2 minutes. Add the stock; bring to a boil. Reduce the heat to medium-low; cover and simmer 10 minutes. Strain stock into a large bowl, discarding the shells.
In the large pot, heat the remaining 1 tbsp. oil over medium. Add the chopped garlic, onion and fennel; season with salt and pepper. Cook, stirring often, until softened, 5 to 6 minutes. Add the tomatoes; bring to a simmer. Cook until the liquid thickens, about 2 minutes. Increase the heat to medium-high; add the stock. Bring to a boil, reduce the heat to medium and simmer, uncovered, until the fennel is tender, about 15 minutes.
In a bowl, mix the mashed garlic, cheese and mayo; spread on the toasts.
Add the shrimp to the pot; cook until just opaque, about 2 minutes. Divide among bowls. Serve with toasts.