In a large pot, mix together strawberries and rhubarb.
Add the sugar, lemon juice, and salt and stir until the fruit is well coated.
Cook over medium-high, stirring frequently, until the fruit comes to a rolling boil.
Then reduce the heat a bit so that the fruit is still boiling but not so rapidly.
Continue cooking for 30-40 minutes while continually scraping around the sides, checking that the bottom isn't burning and skimming off any foam.
You will know when the jam is ready when it passes the "wrinkle test" (Something I learned from David Lebovitz's blog). Place a small plate in the freezer for at least 20 minutes. Take the small plate out of the freezer, place a little dollop of jam in the middle of the plate. Place in back in the freezer for a few minutes and take it back out. Nudge the jam with your finger. If it wrinkles when you do that, the jam is ready. If you have a candy thermometer, great. The jam should reach 220F.
Take the jam off the heat and ladle into mason jars, cover and store in the refrigerator for up to a week.