Bring milk and vinegar to just simmering in a non-aluminum pot over high heat without stirring, monitoring temperature until it reaches 195° Fahrenheit. Avoid boiling, as it risks overcooking the curds. Remove from heat and rest for 10 minutes. More curds will develop and begin to pull away from edges of the pot. Transfer all to a cheesecloth- or tea-towel-lined sieve placed over a large bowl, ladling most of the solids (curds) into the sieve before gently pouring the remaining liquid and curds over the top.
Leave to strain, occasionally stirring to allow liquid (whey) to seep through, until desired consistency is reached. Season ricotta with salt to taste. Reserve they whey for another use.
Sprinkle ricotta with crushed red pepper, sea salt and lemon zest on top, and serve with radishes, cucumbers and crusty sourdough bread.