Make the brine by combining all of the ingredients with 2 litres (70 fl oz/8 cups) of water in a large saucepan over high heat and cook for 1–2 minutes or until the salt and sugar have dissolved. Refrigerate until completely cooled. Once cooled, add the tongue, placing a weight on top to ensure it stays completely submerged in the liquid. Cover and refrigerate for 7 days.
Remove the tongue from the brine, reserving 500 ml (17 fl oz/2 cups) of the liquid, and place in a large saucepan. Cover with water and bring to the boil over high heat. Strain and immediately return to the saucepan, cover with fresh water and reserved brining liquid. Simmer for 3 hours over low to medium heat. To test, pierce the tongue using a skewer at the thickest part and the tip: both should be tender.
Remove the tongue from the saucepan, reserving the cooking liquor, and, while warm, remove the thick leather membrane. Start at the back or throat end of the tongue and use a small knife to lift up the first piece of skin. Then, using your fingers, peel the skin off as though you were removing a glove. Be careful when you reach the tip of the tongue that you don’t tear it off. Discard the skin.
Make the pea ice by blanching and refreshing the peas. Put the peas in a blender with 1–2 tablespoons of the tongue’s cooking liquor and a drizzle of olive oil. Purée until smooth and season generously with salt and pepper. Put the mixture in an ice-cream machine and churn until a fine ice is achieved. Alternatively, you can serve it as a chilled purée.
Thin the mustard with 1–2 tablespoons of the tongue’s cooking liquor. When ready to serve, thinly slice the tongue. Place a piece of tongue on each plate with a little of the mustard, top with a spoonful of pea ice and scatter with the fresh salad leaves.