Toast the bread in a toaster or oven until brown. Set aside.

In a medium skillet, melt half the butter. Add the white ramp bulbs and cook over moderately high heat, stirring, until both the butter and ramps are just starting to brown, 3-4 minutes. Add the green leaves and cook 30 seconds more, until just wilted. Season lightly with salt and pepper. Pour ramps and brown butter into a bowl and set aside.

In the same pan, melt the rest of the butter on medium-high heat. Add the oyster mushrooms in a single layer and let them cook without moving them until one side is deeply caramelized and golden brown, 3-4 minutes. Flip and brown the other side, another 1-2 minutes. Season lightly with salt and pepper, remove from pan and set aside.

Spread each toast slice with 1/4 cup of the ricotta. Arrange the mushrooms and ramps on top, drizzling on a little of the reserved browned butter. Cut each crostini in half and serve.