How to Make It
Line a colander with moistened cheesecloth and set it over a large bowl. In a large saucepan, bring the milk to a gentle boil. Add 3 tablespoons of the lemon juice and cook over low heat, stirring gently, until curds form and rise to the surface, about 3 minutes. Remove from the heat, cover and let stand for 5 minutes; the curds will firm up slightly.
Using a slotted spoon with small holes, gently scoop the curds into the colander; discard the liquid. Let the ricotta drain for 15 minutes, then gently lift the cheesecloth to flip the curd and let drain for 15 minutes longer. Transfer the ricotta to a bowl and season with salt.
Preheat the broiler. Arrange the bread slices on a baking sheet and brush with olive oil. Broil the bread 8 inches from the heat for about 4 minutes, turning once, until golden and crisp.
In a bowl, whisk the remaining 1 tablespoon of lemon juice with the 2 tablespoons of oil; season with salt and pepper. Add the radishes and arugula and toss.
Spread the ricotta generously on the toasts and top with the radishes and arugula. Garnish with pepper and serve.