Toss sliced mango and salt together in a glass or plastic bowl. Cover the bowl tightly in plastic film and store in a sunny area of your kitchen anywhere from 4 hours to five days.
After the salting period has elapsed, strain your mangoes in a colander over a clean bowl. Be sure to save the liquid you strain from them!
Lay out your mango slices on a sheet pan lined with paper towels. Place another layer of paper towels over your mango slices and press to absorb any excess moisture. Allow the mango to dry out in the sun for three to four hours, or transfer them to a clean sheet pan lined with a silicone mat and bake at 170°F for one hour.
While your mangos are drying, bring the reserved mango juice to a boil. Remove from the heat and keep warm.
In a large pan heat 1 1/2 tbsp of the corn oil over medium-high heat. Once the oil begins to ripple, add the mustard seeds, coriander, cumin, dried chili peppers. Once the spices begin to sizzle and pop, add the fenugreek, paprika, and turmeric. Stir frequently to keep the mixture from scorching. Once your spices are fragrant (about two to three minutes), add the mango liquid. Stir to combine. Bring this mixture to a boil, then reduce the heat to simmer. Allow this sauce to reduce until it is a thick sauce (tomato sauce consistency).
Add the dried mango slices to the simmering sauce and stir to combine. Now add the lemon juice and garlic and stir well. Pour a tbsp of water to thin the mixture out and bring to a simmer. Allow this to cook for 15 minutes, or until your mango slices are very soft. If you notice the Amba is becoming too thick, or dry, add more water (a few tablespoons at a time).
Once the mango has softened, remove the pan from the heat and allow it to cool slightly. Pour the mango mixture into a blender and puree until very smooth. Remember to vent your blender's lid and cover it with a folded towel to prevent mishaps. The final sauce should be the consistency of yogurt. If it's too thick add a small amount of water.
Transfer the Amba to glass storage jars. Float two tbsps of corn oil on the surface of each jar of Amba to mix in before serving and refrigerate until cool.
This Amba will keep for up to one month under proper refrigeration.