Has a very big entry in my Complete Book of Vegetables by Matthew Biggs.
Says sow early in the year, may need to cloche/blackpoly the ground for a while before sowing to warm up the earth. Sow in drills 1/2inch deep, 10-12inches apart, thin to final spacing of 9inches. Alternatively sow in stations, 3-4 seeds every 9inches and thin to strongest seedling. Can make second sowing in mid summer to provide early crops the next year.
Dig in plenty of well-rotted compost/organic matter in the autumn before planting or where soil has been manured for the previous crop. Rake finely before sowing, slow germination, keep weed free especially when they're seedlings, hoe carefully to avoid root damage. Mulch to conserve moisture. Can suffer from parsnip canker but is otherwise "trouble-free". If you station sow, you can fill in the spaces with small lettuces like "Tom Thumb". Use radishes as an marker intercropping so you don't weed out the parsley seedlings.
"Warning: Hamburg parsley should not be eaten in large amounts by expectant mothers or those with kidney problems"
Culinary... flavour reminiscent of celeriac and is frequently used in eastern european cooking. Scrub clean gently - don't peel or scrape. Can be roasted, sauteed or fried like chips, boiled, steamed or added to soups/stews, can also be grated raw into winter salads. Can be dried in a low oven and used in stews as flavouring. Has a recipe for Croatian Hamburger Parsley Soup.
Medicinal, leaves are good source of vitamins A & C. Reduces inflammation, used for eurological conditions like cystitis and kidney stones and helps indigestion, arthritis, rheumatism. After childbirth it encourages laction... the roots and seeds promote uterine contractions (hence previous warning), leaves are excellent breath freshner and can counteract garlic breath