Place the whole head of garlic in an oiled roasting tin - I used a little pannacotta mould that the garlic could sit snugly in – and drizzle a little olive oil over the garlic. Pop the garlic in a preheated 350F (180C) oven, and roast it for 30 minutes.
Meanwhile, wash and drain the lentils, and then place them in a large, heavy-bottomed, preferably non-stick pan, with the 3 ¾ cups of water. Bring to the boil, then reduce heat to a simmer, partially covering the pan with its lid. Stirring occasionally, allow the lentils to simmer for about 35 minutes, or until the mixture starts to take the appearance of very thick soup. You may need to add some water to the pan, from time to time, as it evaporates and the mixture starts to look dry.
Once you have the lentils on to cook, melt the ghee or butter in a small to medium skillet, and cook the onion, fresh chilies and ginger over low heat, stirring from time to time, until golden – about 10 minutes. Just as the lentils are becoming tender, add the onion mixture to the lentils and stir to combine. Then allow the lentils to finish cooking. Give the skillet a bit of a wipe with kitchen paper, as you will use it again shortly.
At about this time, the garlic should be soft and tender. Remove it from the oven and let it cool – roasted garlic can be extremely hot, so take care. Cut off the top third of the head, and holding the garlic over a small bowl, squeeze and otherwise dig out the flesh from each clove. Next, mash the garlic with the 2 tablespoons olive oil.
Add the roasted garlic puree, ground cumin and ground coriander to the lentil mixture, and season with salt and pepper to taste. Cook for 10 minutes, stirring frequently, then add the diced tomatoes and a little lemon juice, to taste. Mix thoroughly, then reduce the heat while you prepare the spice mix.
Heat the oil in the skillet and fry the sliced shallots until quite crispy and brown. Add the garlic and cook until it just colours slightly – overcooked garlic gets bitter. Remove this mixture from the pan and set it aside.
Melt the ghee or butter in the same pan, and fry the cumin and mustard seeds until the mustard seeds start popping. Stir in the dried chilies, curry leaves and the shallot mixture, then tip it all into the lentils and swirl it around.
Garnish the dhal with the fresh cilantro and serve immediately.