2 pounds firm celery stalks, medium
1 cup sugar
¼ teaspoon fine sea salt
½ cup lemon juice, freshly squeezed (from about 4 lemons)
Rinse, dry and trim the celery stalks. Shave off any tough and stringy outer peel with a vegetable peeler. Slice the stalks crosswise in half; slice all these chunks lengthwise into very thin sticks, about 1/8-inch thick. Cut the sticks crosswise into 1/8-inch cubes and bits (the size of pickle relish). You should have 6 to 7 cups of fine celery pieces.
Put the cup of sugar and the salt in the saucepan, pour the lemon juice on top and then all celery. Set the pot over medium- low heat and stir as the sugar dissolves and the celery heats and starts releasing its juices. Bring the syrup to a gentle simmer, stirring frequently.
Cover the pot and adjust the heat to maintain a gently bubbling around the edges of the pan. Cook for about 15 minutes, stirring now and then. Uncover the pot and cook at the simmer, stirring occasionally, until almost all of the liquid has been absorbed or evaporated, about 35 minutes or more.
Remove the pan from the heat and let the mostarda cool completely before using. Store in a sealed container in the refrigerator.