In a large heavy-bottomed pot or Dutch oven, heat the olive oil over medium-high heat.
Add the whole cumin and fenugreek seeds and toast until fragrant, about 1 minute.
Add the rinsed red lentils, chopped onion, grated ginger, garlic, turmeric, chili pepper, 4 cups / 950 ml of water, and salt.
Stir to combine.
Bring the mixture to a boil, then immediately reduce the heat to maintain a gentle simmer.
Skim and discard any foam that rises to the surface as the lentils begin to cook.
Once the foam subsides, continue to simmer uncovered, stirring occasionally, until the lentils are very tender and have broken down, about 30 to 40 minutes.
Turn off the heat.
Stir in the baby spinach and fresh lemon juice until the spinach has just wilted into the hot dal.
Ladle into shallow bowls and serve immediately.
Don't skip toasting the whole spices. This step, called tempering or tadka, is crucial for unlocking their deep, aromatic flavors and is a hallmark of Indian cooking.